Tuning the index of a waveguide structure

ABSTRACT

The index of refraction of waveguide structures can be varied by altering carrier concentration. The waveguides preferably comprise semiconductors like silicon that are substantially optically transmissive at certain wavelengths. Variation of the carrier density in these semiconductors may be effectuated by inducing an electric field within the semiconductor for example by apply a voltage to electrodes associated with the semiconductor. Variable control of the index of refraction may be used to implement a variety of functionalites including, but not limited to, tunable waveguide gratings and resonant cavities, switchable couplers, modulators, and optical switches.

PRIORITY APPLICATION

This application claims priority under 35 U.S.C. § 119(e) from U.S.Provisional Patent Application Ser. No. 60/318,486, entitled “TunableResonant Cavity Based on the Field Effect In Semiconductors,” filed Sep.10, 2001, U.S. Provisional Patent Application Ser. No. 60/327,137, “HighSpeed Optical Modulator Based on CMOS Compatible Tunable ResonantCavity” filed Oct. 4, 2001, and U.S. Provisional Application Ser. No.60/328,474, entitled “Technique for Tuning the Index of an OpticalStructure and Use of this Effect for Tuning the Coupling,” filed Oct.11, 2001, as well as U.S. Provisional Patent Application Ser. No.60/318,445 entitled “SOI Waveguide with Polysilicon Gate” and filed Sep.10, 2001, each of which is hereby incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to semiconductor devices, and moreparticularly to controlling the propagation of photons throughsemiconductor structures.

2. Description of the Related Art

Light offers many advantages when used as a medium for propagatinginformation, the foremost of which are increased speed and bandwidth. Incomparison with electrical signals, signals transmitted optically can beswitched and modulated faster and can include an even greater number ofseparate channels multiplexed together. Accordingly, lightwavetransmission along optical fibers is widespread in thetelecommunications industry. In an exemplary fiber optic communicationsystem, a continuous wave (CW) beam of light may be emitted from a laserdiode and modulated using an electro-optical modulator that is driven byan electrical signal. This electrical signal may correspond to voice ordata which is to be transmitted over a distance between, e.g., twocomponents in a computer, two computers in a network, or two phonesacross the country or the world. The light travels in an optical fiberto a location where it is detected by an optical sensor, which outputsvoltage that varies in accordance with the modulation of the opticalbeam. In this manner, information can be rapidly transported from onelocation to another. To increase data throughput numerous opticalsignals at different wavelengths can be multiplexed and transmittedtogether along a single optical path. This optical path can be switchedselectively and varied to direct the optical signals to the appropriatedestination.

In constructing optical systems, such as the one described above, avariety of functionalities are desirable. One useful element is amodulator for varying a specific property of light such as amplitude orphase. Another valuable component is a tunable filter for selectivelyisolating certain optical frequencies. Additional useful elements arecouplers and switches for controllably transferring light from one pathto another. What is needed are advantageous designs and techniques formodulating and filtering light as well as for coupling and switchingoptical signals from one path to another.

SUMMARY OF THE INVENTION

In one aspect of the invention, an apparatus comprises a waveguide, atunable resonant cavity, and first and second electrodes. The tunableresonant cavity comprises a closed path for propagating electromagneticwaves, the close path comprising a semiconductor having a distributionof free carriers. The closed path is juxtaposed with the waveguide topermit the coupling of electromagnetic waves between the waveguide andthe closed path. The first and second electrodes are positioned to applyan electric field through an insulator into the semiconductor of thetunable resonant cavity. The distribution of free carriers in thesemiconductor is responsive to the electric field to vary phase delayintroduced by the closed path.

In another aspect of the invention, an optical apparatus also comprisesa waveguide, a tunable resonant cavity, and first and second electrodes.The tunable resonant cavity comprises a semiconductor having adistribution of free carriers and a substantially circular optical path.The circular optical path is juxtaposed with the waveguide to permit thecoupling of light between the waveguide and the circular optical path.The first and second electrodes are positioned to apply an electricfield through an insulator into the circular optical path. Thedistribution of free carriers in the circular optical path is responsiveto the electric field to vary the optical path length of the circularoptical path.

Still another aspect of the invention comprises a method of tuning aresonant cavity. In this method an optical resonator comprisingsemiconductor is provided and an electric field is applied through aninsulator to at least a portion of the semiconductor to alter freecarrier distribution in said semiconductor. The resonant frequency ofthe optical resonator is thereby changed from a first frequency to asecond frequency.

Yet another aspect of the invention comprises an optical apparatuscomprising a first waveguide, a second waveguide, a semiconductor andfirst and second electrodes for applying an electric field through aninsulator to the semiconductor. The semiconductor provides an opticalpath between the first and second waveguides to couple light between thewaveguides. The adjustment of the electric field changes the freecarrier density in the optical path such that absorption of light in theoptical path increases, thereby decreasing the coupling of light betweenthe first and second waveguides.

Another aspect of the invention comprises an optical switching apparatuscomprising first and second waveguides and a carrier controlled opticalswitch having at least first and second states. The carrier controlledoptical switch comprises a coupling waveguide which provides an opticalpath between said first and second waveguides and first and secondelectrodes. The coupling waveguide provides an optical path between thefirst and second waveguides. The coupling waveguide comprises asemiconductor having a refractive index dependent on a distribution offree carriers within the semiconductor. The first and second electrodesare for applying an electric field through an insulator into thesemiconductor. The distribution of free carriers is responsive toapplication of the electric field to change the state of the carriercontrolled optical switch from the first state to the second state.

Another aspect of the invention comprises a method of selectivelycoupling light between first and second waveguides. The method comprisesproviding a semiconductor positioned to couple light between thewaveguides along an optical path and changing free carrier density ofthe semiconductor in the optical path to alter coupling between thewaveguides.

Still another aspect of the invention comprises an optical waveguidecoupler having a tunable coupling coefficient. The coupler comprises afirst waveguide and a second waveguide juxtaposed for coupling and afirst electrode. The first waveguide is comprised of semiconductorhaving a distribution of free carriers. The first electrode iselectrically connected to a first variable voltage source for applyingan electric field to the semiconductor of the first waveguide. Thedistribution of free carriers is responsive to application of theelectric field to change the coupling coefficient between the waveguidesfrom a first value to a second value.

Still another aspect of the invention comprises a method of tuning thecoupling coefficient of an optical waveguide coupler. In this method, afirst waveguide comprising semiconductor containing a distribution offree carriers is provided. A second waveguide juxtaposed to the firstwaveguide is also provided. An electric field is applied to thesemiconductor of the first waveguide to alter the free carrierdistribution in the semiconductor, thereby changing the couplingcoefficient of the optical waveguide coupler from a first value to asecond value.

Yet another aspect of the invention comprises a waveguide gratingcomprising a waveguide for propagating light in a longitudinaldirection. The waveguide comprises a plurality of elongate membersoriented transverse to the longitudinal direction. The members aredisposed relative to the waveguide to form a grating for coupling lightout of the waveguide. The waveguide has a carrier density at each of themembers. These members include respective electrodes for applying anelectric field to the waveguide, the electric field varying this carrierdensity in the waveguide such that the coupling is altered.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are described below inconnection with the accompanying drawings.

FIG. 1 is a schematic illustration of an embodiment of an opticalswitching apparatus including a carrier controlled optical switchcomprising a resonant cavity disposed between two waveguides.

FIG. 2 is a cross-sectional view of a preferred embodiment of a resonantoptical cavity formed with a disk-shaped semiconductor.

FIGS. 3 and 4 are top and perspective views, respectively, of theresonant optical cavity of FIG. 2.

FIG. 5 is a cross-sectional view of a preferred embodiment of a resonantoptical cavity formed with an annular-shaped semiconductor.

FIGS. 6 and 7 are a top and perspective views, respectively, of theresonant optical cavity of FIG. 5.

FIGS. 8 and 9 are top views showing different electrode configurationsassociated with preferred embodiments of a resonant optical cavity.

FIGS. 10A and 10B are perspective and cross-sectional views of aresonant optical cavity schematically illustrating confinement of lightin the optical cavity.

FIGS. 10C and 10D are cross-sectional views of the resonant opticalcavity depicted in FIGS. 10A and 10B schematically illustrating opticalconfinement by introducing an annular shaped strip around the perimeterof the optical cavity and doping the center of the optical cavity.

FIG. 11 is a cross-sectional view of another resonant optical cavityformed with a disk-shaped semiconductor configured to provide analternative carrier distribution.

FIG. 12 is a plot on axis of frequency (in arbitrary units) and opticalpower (in arbitrary units) depicting the quality factor associated withdifferent states of a resonant optical cavity.

FIG. 13 shows plots illustrating the output variation over time in firstand second waveguides responsive to a modulation voltage applied to anoptical switching apparatus such as depicted in FIG. 1.

FIG. 14A is a perspective view of a directional coupler comprising apair of spatially separated waveguides brought within close proximityalong a coupling region. FIG. 14B shows a cross-sectional view throughthe line 14B—14B through the coupling region of the directional couplerof FIG. 14A.

FIGS. 15A–15C are plots of light intensity as a function of locationalong the line 15—15 in FIG. 14B illustrating the extent of coupling forthree different optical states.

FIGS. 16–19 are cross-sectional views of waveguides pairs comprising adirectional coupler such as shown in FIG. 14A each configureddifferently to selectively alter the optical states for varied levels ofoptical coupling.

FIG. 20A shows a schematic top view of a waveguide adjacent to adisk-shaped optical resonator. FIG. 20B shows a cross-sectional view ofthe waveguide and optical resonator along a line 20B—20B in FIG. 20A.

FIG. 21 is a perspective view of a waveguide grating comprising aplurality of electroded rulings on a channel waveguide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

These and other embodiments of the present invention will also becomereadily apparent to those skilled in the art from the following detaileddescription of the preferred embodiments having reference to theattached figures, the invention not being limited to any particularembodiment(s) disclosed. Accordingly, the scope of the present inventionis intended to be defined only by reference to the appended claims.

In general, optical waveguides comprise a core region comprisingmaterial that is at least partially transparent. This core region issurrounded by a cladding region that confines light within the coreregion. Some optical energy, often referred to as the evanescent energyor the evanescent field, however, may exist outside the core region andwithin the cladding region.

In certain waveguides, the core region comprises a first medium having afirst refractive index, and the cladding region or cladding comprises asecond medium having a second refractive index, the refractive index ofthe core region being greater than the refractive index of the claddingregion. A core/cladding interface is located at the boundary between thecore region and the cladding region. In such embodiments, when light inthe core region is incident upon this core/cladding interface at anangle greater than the critical angle, the light is reflected back intothe core region. This effect is referred to as total internalreflection. In this manner, optical signals can be confined within thecore region due to total internal reflection at the core/claddinginterface.

Waveguides can be fabricated in a wide variety of geometries andconfigurations. A channel waveguide, and more specifically, a buriedchannel or embedded strip waveguide, is a specific type of waveguidethat fits the description above. A channel waveguide generally comprisesa core comprising a first medium having a relatively high refractiveindex surrounded by a relatively lower refractive index cladding region.A buried channel or embedded strip waveguide generally comprises a coreembedded in a substrate that forms at least part of the surroundingcladding region.

A buried channel waveguide is an example of an integrated opticalwaveguide, which are generally associated with a substrate. Theintegrated optical waveguide may for example be situated on thesubstrate, in a substrate, or partially on and partially in thesubstrate. The integrated optical waveguide may be part of the substrateitself but preferably comprises one or more layers of materialpositioned on a surface of the substrate. Examples of integrated opticalwaveguides include the channel waveguides discussed above, as well asslab waveguides, rib or ridge waveguides, and strip loaded waveguides.

In accordance with conventional usage in the art, optical componentsthat are integrated onto a substrate with integrated optical waveguides,are collectively referred to herein as integrated optics. Such opticalcomponent may for example, process, manipulate, filter or otherwisealter or control optical signals propagating within the waveguides. Asdiscussed above, these components themselves may be waveguides thatguide light.

One of the simplest integrated optical waveguide configurations is theconventional slab waveguide. The slab waveguide comprises a thin, planarslab surrounded by cladding regions. The cladding regions may take theform of first and second (for example, upper and lower) cladding layerson either side of the slab. The two cladding layers need not comprisethe same material. In this simplified example, the slab may be planarwith substantially parallel planar boundaries at the interfaces with thefirst and second cladding layers. Generally, the slab has a higherrefractive index than either of the cladding layers. Light can thereforebe confined in one dimension (e.g., vertically) within the slab. In thisconfiguration of the slab waveguide, optical energy is not confinedlaterally to any portion of the slab, but extends throughout the slabdue to total internal reflection at the planar boundaries between theslab and the surrounding upper and lower cladding layers.

A ridge or rib waveguide is formed by creating thickness variations inthe slab. These thickness variations may be formed by depositingmaterial on selected regions of the slab or by removing material fromselected regions of the slab. The slab with the ridges or ribs formedthereon may be surrounded on opposite sides by the first and second(e.g., upper and lower cladding layers) comprising relatively lowrefractive index material. The thicker portions, i.e., the ridges orribs, which comprise more slab material, will have a higher effectiveindex than thinner region of the slab which comprise relatively lesseramounts of the slab material.

Accordingly, the region within the slab that is beneath the thickerportions and in proximity thereto has a higher effective refractiveindex than other portions of the slab. Thus, unlike the slab waveguidewherein optical energy propagates throughout the planar slab, the ridgeor rib waveguide substantially confines optical energy to the region ofthe planar slab layer within and under the ridge and in proximitythereto. In a ridge or rib waveguide, therefore, an optical signal canbe propagated along a path in the slab defined by the region under whichthe ridge or rib is located on the slab. Thus, ridge waveguides definingany number and variations of optical pathways can be created by formingone or more ridges or ribs in the slab having the shape and orientationof the desired optical pathways.

Similarly, a strip loaded waveguide is formed by positioning a strip onthe slab of a slab waveguide. The slab and the strip located thereon maybe surrounded on opposite sides by the first and second (e.g., upper andlower) cladding layers having lower refractive index than the slab.Preferably, the strip has a refractive index that is greater than thatof either cladding layer, however, the index of the strip is preferablyapproximately equal to that of the slab. The presence of the strippositioned on the slab induces an increase in effective index of theslab in the region beneath the strip and in proximity thereto.

As with the ridge or rib waveguide, the region within the slab that isbeneath the strip and in proximity thereto has a higher effectiverefractive index than other portions of the slab. Thus, the strip loadedwaveguide substantially can confine optical energy to the region of theplanar slab layer under the high-index strip, some of the optical energyalso being within the strip itself. Accordingly, in a strip loadedwaveguide an optical signal can be propagated along a path in the slabdefined by the region over which the high-index strip is placed on theslab. Waveguides corresponding any number and variations of opticalpathways, can be created by depositing one or more strips onto the slabhaving the shape and orientation of the desired optical pathways.

Another form of waveguide discussed in U.S. patent application Ser. No.10/241,284 entitled “Strip Loaded Waveguide with Low-Index TransitionLayer” filed Sep. 9, 2002, which is hereby incorporated herein byreference in its entirety, comprises a slab having a first refractiveindex n₁ and a strip having a second refractive index n₂. In addition,the strip loaded waveguide structure has a transition layer having athird refractive index n₃. The transition layer is positioned betweenthe slab and the strip, such that the slab and the strip do not directlycontact each other. The refractive index of the transition layer n₃ isless than the refractive index of the slab n₁ and the refractive indexof the strip n₂.

The light within the slab is confined to portions beneath the stripbecause of the presence of the strip, despite the fact that the strip isseparated from the slab. The intervening transition layer does notprevent the strip from determining the shape and location of the opticalmode(s) supported in the slab. The presence of the strip positionedproximally to the slab portion induces an increase in effective index ofthe slab portion in the region directly under the strip and in proximitythereto. This increase in effective index defines a relatively higheffective index guiding region wherein light in one or more supportedoptical modes is guided along the strip loaded waveguide. The striploaded waveguide guides supported modes in the guiding region despitethe presence of the transition layer between the slab and strip. Inparticular, the transition layer does not prevent the strip fromaltering the effective index within the slab and more particularly, fromraising the effective index within the slab. Preferably, the transitionlayer has a thickness sufficiently small such that the strip canincrease the effective index of the slab in regions immediately beneathand in the proximity thereto. The transition layer is sufficiently thinand the strip and the slab are sufficiently close, although physicallyseparated by the intervening transition layer, that the strip can affectthe propagation of light within the slab. The transition layer alsopreferably has an index of refraction that is low in comparison withthat of the strip and the slab.

In certain embodiments of the invention, semiconductor materials used inconventional processes for fabrication of semiconductor microelectronicsare employed to create waveguide structures. These materials include,but are not limited to, crystalline silicon, polysilicon and silicondioxide (SiO₂). In particular, in various preferred embodiments of thestrip load waveguide having an insulating transition layer, the slabcomprises single crystal silicon, the transition layer comprises silicondioxide, and the strip comprises polysilicon, although in otherembodiments, the strip may comprise crystal silicon. The crystal siliconslab and the polysilicon strip are preferably doped so as to beelectrically conducting although in portions of the slab or strip thatare not to be conductive, the slab and the strip are preferably undopedto minimize absorption losses.

As is well known, single crystal silicon is used to fabricatesemiconductor microelectronics and integrated circuits (ICs), such asmicroprocessors, memory chips and other digital as well as analog ICs,and thus single crystal silicon is well characterized and its propertiesare largely well understood. The term single crystal silicon is usedherein consistently with its conventional meaning. Single crystalsilicon corresponds to crystalline silicon. Single crystal silicon,although crystalline, may include defects such that it is not truly aperfect crystal, however, silicon having the properties conventionallyassociated with single crystal silicon will be referred to herein assingle crystal silicon despite the presence of such defects. The singlecrystal silicon may be doped either p or n as is conventional.

Single crystal silicon should be distinguished from polysilicon or“poly”. Polysilicon is also used to fabricate semiconductormicroelectronics and integrated circuits. The term polysilicon or “poly”is used herein consistently with its conventional meaning. Polysiliconcorresponds to polycrystalline silicon, silicon having a plurality ofseparate crystalline domains. Polysilicon can readily be deposited forexample by CVD or sputtering techniques, but formation of polysliconlayers and structures is not to be limited to these methods alone.Polysilicon can also be doped p or n and can thereby be madesubstantially conductive. In general, however, bulk polysilicon exhibitsmore absorption losses in the near infrared portion of the spectrum thana similar bulk single crystal silicon, provided that the doping,temperature, and other parameters are similar.

Optical switches, modulators, and couplers, among other devices, can beimplemented using various waveguide structures including but not limitedto the types discussed above, e.g., channel, slab, rib or ridge,strip-loaded, and strip loaded with transition layer. In addition, thesestructures can be formed using semiconductor materials, such as forexample, silicon.

A. Optical Switching Apparatus

FIG. 1 is a schematic diagram of an optical switching apparatus. Theswitching apparatus includes a carrier controlled optical switch 104that may be used to couple light between a first waveguide 100 and asecond waveguide 102.

The effective refractive index of the first and second waveguides 100,102 is larger than the effective refractive index of cladding regions108 surrounding the waveguides so as to allow the waveguides 100, 102 topropagate light in a guided fashion, as discussed above.

The carrier controlled optical switch 104 includes an optical pathbetween the first waveguide 100 and the second waveguide 102. In variouspreferred embodiments, the optical path comprises a resonant cavity 106,preferably comprised of an optically transparent semiconductor. Moreparticularly, this resonant cavity 106 preferably comprises a waveguidestructure comprising semiconductor material. The optical path furthercomprises a first gap region, A, between the first waveguide 100 and theresonant cavity 106, and a second gap region, B, between the secondwaveguide 102 and the resonant cavity 106. The sizes of the gap regions,A, B, permits control of the coupling of light between waveguides 100,102 and the resonant cavity 106, and allows for a weak coupling oflight, which is desirable under certain conditions.

Preferably, the resonant cavity 106 is configured to accumulate ordeplete free carriers such as electrons and/or holes. The refractiveindex of the material comprising the resonant cavity 106 issignificantly larger than the refractive index of the confining region108 enabling light to be guided within the resonant cavity. Furthermore,the refractive index of at least a particular region within thesemiconductor 106 is variable, depending upon the density of freecarriers in that region.

The resonant cavity 106 further includes an electrode 110 for applyingan electric field through an insulator 112 into the semiconductor 106.(The insulator preferably comprises silicon dioxide.) The electrode 110is preferably metal or polysilicon, and is connected to a variablevoltage source 114 that may be used to control the magnitude of anelectric field applied to the semiconductor 106.

These waveguides 100, 102 as well as the resonant cavity 106 depictedschematically in FIG. 1, may comprise channel waveguides, rib or ridgewaveguides, or strip loaded waveguides although the waveguide designshould not be limited to these specific types. In one preferredembodiment, however, the waveguides 100, 102 comprise strip loadedwaveguides having a low-index transition layer between the strip and theslab described above as well as disclosed in in U.S. patent applicationSer. No. 10/241,284 entitled “Strip Loaded Waveguide with Low-IndexTransition Layer” filed Sep. 9, 2002.

These particular strip loaded waveguides comprises comprises a slab anda strip, wherein the strip is separated from the slab. A layer ofmaterial having an index of refraction lower than that of the strip andthe slab is disposed between and separates the strip and the slab.Nevertheless, a guiding region is provided for propagating an opticalmode and this guiding region extends both within the strip and the slab.In certain embodiments, for example, the slab and strip comprisesemiconductor and the transition region comprises dielectric.

Application of a voltage between the semiconductor strip and the slabcauses carriers to accumulate within the guiding region of the striploaded waveguide. For example, depending on the polarity of the appliedvoltage and the doping, electrons or holes may accumulated or bedepleted within the semiconductor slab in a regions adjacent to the thintransition layer comprising dielectric material. The structure acts likea capacitor, charging with application of a voltage. The voltage createsan electric field across the thin transition layer with carriersaccumulating (or depleting) adjacent to this transition layer.

These strip loaded waveguides are preferably located on a supportingstructure or substrate. The supporting structure serves to support thestrip loaded waveguide and preferably comprises a material such assilicon or sapphire. Additionally, the supporting structure may alsoinclude a cladding layer or layers, which aid in confining opticalenergy within the slab portion. Accordingly, this cladding preferablyhas a refractive index that is low in comparison to the refractive indexof the slab.

In one preferred embodiment, the supporting structure comprises asilicon substrate having a cladding layer of silicon dioxide formedthereon. The silicon dioxide layer on the silicon substrate with anindex of approximately 1.5 serves as a lower cladding layer for theslab. This silicon substrate may be doped.

Accordingly, the slab is disposed either on the substrate or on a layer(preferably the cladding) formed over the substrate. This cladding layeritself may be formed directly on the substrate or may be on one or morelayers formed on the substrate. As discussed above, the slab preferablycomprises single crystal silicon and has an index of refraction n₁ onaverage of about 3.5 and a thickness t₁ preferably between about

$\frac{\lambda}{6\; n}$and

$\frac{\lambda}{4n},$and more preferably about

$\frac{\lambda}{4n}.$This thickness, t₁, determines in part the optical mode or modessupported by the strip loaded waveguide and depends partially on thegeometry of the structure. In alternative embodiments, the slab maycomprise materials other than single crystal silicon and may be doped orundoped and thus may have different refractive indices. The slab,however, preferably comprises crystal silicon. Localized doping, such asused to create the source, drain, and channel regions in a transistor,may cause the index of refraction in localized regions of the slab tovary slightly.

In general, the strip is disposed above and in a spaced-apartconfiguration with respect to the slab. The strip may comprise dopedpolycrystalline silicon having an index of refraction n₂ ofapproximately 3.5. In alternative embodiments, the strip may comprisedoped single crystal silicon having an index of refraction n₂ on averageabout 3.5. As discussed above, however, the strip may also be undopedand may comprise materials other than polysilicon or crystal siliconalthough these materials are preferred. An example of one suchalternative material that may be used to form the strip is siliconnitride, which has an index of refraction of approximately 1.9.

The dimensions of the strip may vary and depend in part on the overallcomposition and geometry of the waveguide. As with the slab, the size ofthe strip determines in part the number of modes to be supported by thewaveguide and the wavelength of these modes.

The transition layer is positioned between the slab and the strip.Preferably, the refractive index of the transition layer is less thanthe refractive index of the polysilicon strip and the crystallinesilicon slab. In one preferred embodiment, the transition layercomprises silicon dioxide having an index of refraction n₃ ofapproximately 1.5.

The strip loaded waveguide is preferably covered at least partially by acoating although more than one coating or layers may be formed on thewaveguide in various embodiments. This coating may provide electricalinsulation between separate conductive pathways. The coating may alsoserve as a cladding layer, providing confinement of optical energywithin the slab and the strip. Accordingly, the coating or coatingspreferably has a composite index of refraction lower than that of theslab and the strip. The coating may have an index of refraction equal tothat of the low-index transition layer and may comprise the samematerial as the low-index transition layer. Alternatively, the coatingmay have a different index of refraction than the transition layer andmay comprise different material. The coating preferably comprisessilicon dioxide. Other materials and, more specifically, otherdielectrics may also be employed.

Confinement of light within the slab is provided because the slab has ahigher refractive index than the layers above and below. In onepreferred embodiment, for example, light is confined within the siliconslab because the silicon slab has a higher refractive index than thesilicon dioxide coating covering it. In addition, the silicon slab has ahigher index than the silicon dioxide cladding layer immediately belowit. Lateral confinement within the slab is provided by the loadingcaused by the strip.

In this manner, light can be propagated through specific guiding regionswithin the slab. The guiding region corresponds to a boundary where aspecific portion of the optical energy within the mode, preferably thefundamental mode, is substantially contained and thus characterizes theshape and spatial distribution of optical energy in this mode.Accordingly, the guiding region corresponds to the shape and location ofthe optical mode or modes in this strip loaded waveguide. In the guidingregion, the electric field and the optical intensity are oscillatory,whereas beyond the guiding region, the evanescent field exponentiallydecays.

As discussed above, these strip loaded waveguides may be employed toform resonant optical cavities, however, the resonant optical cavitiesdisclosed herein are only exemplary and different designs and materialsystems, both those well known or yet to be devised, may be utilized inthe alternative to create resonant optical cavities, modulators,couplers, switches or other related components.

B. Resonant Optical Cavity

A preferred embodiment of the resonant cavity 106 is illustrated inFIGS. 2 through 4. FIG. 2 is vertical cross-section through the line 2—2shown in FIG. 3. FIGS. 3 and 4 are top and perspective views of theresonant cavity 106, respectively. FIGS. 2, 5, 11, and 20, depictdesigns that include conformal metalization. Alternatively,planarization techniques can be used as is conventional in contemporarysemiconductor device fabrication.

The resonant cavity 106 comprises a disk-shaped slab 204 on top of acladding layer 202 formed on a substrate 200. The disk-shaped slab 204preferably has higher index of refraction than the cladding layer 202.In one preferred embodiment, the disk-shaped slab comprises crystalsilicon (e.g., active crystal silicon) and the cladding layer 202comprises a silicon dioxide layer (e.g., a buried-oxide layer) on asilicon substrate.

An insulating layer 206 covers the disk-shaped slab 204. The insulatinglayer 206 preferably has a refractive index lower than the refractiveindex of the disk-shaped slab 204, so as to act as an upper claddinglayer confining light within the disk-shaped slab 204. The insulatinglayer 206 preferably comprises silicon dioxide, which has a refractiveindex substantially lower than the refractive index of single crystalsilicon. The insulating layer 206 also prevents unwanted flow ofelectrical current between conducting elements of the device. Theinsulating layer 206 may comprise a plurality of layers, preferably lowindex dielectrics films overlaying on each other. Those of skill in theart would recognize that other insulating materials such as polymerslike polyamide may be used, provided they have appropriate opticalproperties.

An annular strip 208 comprising relatively high refractive indexmaterial is disposed over but space apart from the slap 204. Thisannular strip 208 follows a path around the outer portion of thedisk-shaped slab 204. The annular strip 208 preferably comprisesmaterial having an index of refraction that is high compared to that ofthe insulating layer 206 covering the disk-shaped slab 204. Thismaterial comprising the strip 208 is also preferably substantiallytransparent and non-absorbing to the wavelength light for which theresonant cavity 106 is designed. Preferably, the strip material issubstantially conductive and may comprise doped semiconductor. In onepreferred embodiment, the annular strip 208 comprises doped polysilicon,which has a refractive index comparable to that of single crystalsilicon. Alternatively, the strip 208 may comprise single crystalsilicon. Those of skill in the art would recognize that other materialsmay be used for the strip 208. The materials preferably have asubstantially high refractive index in comparison with the insulatingmaterial covering the disk-shaped slab 204.

The strip 208 is separated from the disk-shaped slab 204 by a transitionlayer 216 of the insulating material to prevent the flow of currentbetween the strip 208 and the disk-shaped slab 204 to thereby facilitatecarrier accumulation and depletion. This insulating material preferablyhas a lower index of refraction than the disk-shaped slab 204 as well asthe annular shaped strip 208. This transition layer 216 preferably hassufficiently thickness such that the carriers do not traverse thisbarrier either through defects (e.g., “pin hole” defects) or bytunneling. Conversely, the thickness of this dielectric layer 216 ispreferably not so large as to require an excessively highly voltage tobe applied to the device to generate or deplete the desired amount ofcarriers. In one preferred embodiment, this transition layer 206comprises silicon dioxide.

As shown in FIG. 2, the resonant cavity 106 further includes a first(strip) electrode 210 electrically connected to the annular strip 208.As shown in FIGS. 3 and 4, the first electrode 210 includes asubstantially annular portion that is electrically connected to avoltage source 220. This voltage source may be an AC or DC voltagesupply depending on the particular application. This embodiment furtherincludes a second (slab) electrode 212 electrically coupled to a centraltop surface of the disk-shaped slab 204. This central portion of thedisk-shaped slab 204 preferably includes a doped region 214 electricallycontacting the slab electrode 212 so as to create an ohmic contactbetween the disk-shaped slab 204 and the slab electrode 212. Asillustrated in FIG. 4, the second electrode 212 is also electricallycoupled to the voltage source 220 allowing for the application of apotential difference between the strip and slab electrodes 210, 212although other configurations for establishing an electric field acrossthe transition layer 216 are possible. The strip and slab electrodes210, 212 are separated by the insulating layer 206 to prevent unwantedelectrical contact therebetween; see FIG. 2. The insulating layer 206 isnot shown in FIGS. 3 and 4 in order to allow illustration of theinterior features of the resonant cavity structure 106. The strip andslab electrodes 210, 212 preferably are comprised of metal, although oneof skill in the art would recognize that other materials, such as dopedpolysilicon, may be used. Salicide may also be included to formed afavorable electrical contact to semiconductor regions. In particular,ohmic contacts can be formed between a metal electrode and an underlyingsalicide region in the semiconductor. In this manner, for example, theslab electrode 212 can be electrically connected to the semiconductorslab 204.

As discussed above, the strip 208 will confine the light to regionswithin the slab 204 beneath the strip and in proximity thereto as aresult of the effect of the strip on the effective index of the slab. Aportion of the optical power will also be contained within the strip 208as well as the transition layer 216. The thickness of the slab 204 andthe strip 208 as well as the width of the strip will in part determinethe optical mode or modes that are supported, their spatial extent, andthe associated wavelengths. Preferably, these dimensions are selected soas to support a single mode such as the “whispering gallery” mode whichtravels within the disk shaped slab 204 around its perimeter.

The first electrode 210 is also preferably about as wide as the width ofthe optical mode confined below the strip 208. The surface area of thefirst electrode 210 for a resonator with a free spectral rangeequivalent to approximately a 50 nanometers (nm) optical communicationsband and supporting the resonator mode for a given wavelength is roughlya few square microns. This small size for the first electrode 210 allowsfor very high speed modulation due to the small associated capacitance.

The structure 106 shown in FIGS. 2–4 may be manufactured usingconventional integrated circuit fabrication processes. For instance, thesupporting structure may comprise a commercially available silicon waferwith silicon dioxide formed thereon. Conventional “Silicon-on Oxide”(SOI) processes can be employed to form the silicon slab on the siliconwafer or on a sapphire substrate. Fabrication techniques for forming acrystal silicon layer adjacent an insulator include, but are not limitedto, bonding the crystal silicon on oxide, SIMOX (i.e., use of ionimplantation to form oxide in a region of single crystal silicon), orgrowing silicon on sapphire. Oxide formation on the silicon slab can beachieved with conventional techniques used in field effect transistor(FET) technology for growing gate oxides on a silicon active layers.Still other processes utilized in fabricating FETs can also be applied.In the same fashion that a polysilicon gate is formed on the gate oxidein field effect transistors, likewise, a polysilicon strip can be formedover the oxide transition region in the waveguide structure. Thispolysilicon strip can be patterned using well-known techniques such asphotolithography and etching. Damascene processes are also consideredpossible. Accordingly, conventional processes such as those employed inthe fabrication of Complementary Metal Oxide Semiconductor (CMOS)transistors can be used to create the resonant cavity 106. In otherembodiments, crystalline silicon strips can be formed on the transitionoxide region using conventional techniques such as SOI processing.

Another strategy for fabricating the strip loaded waveguide is to obtaina commercially available SOI wafer which comprises a first siliconsubstrate having a first silicon dioxide layer thereon with a secondlayer of silicon on the first silicon dioxide layer. The aggregatestructure therefore corresponds to Si/SiO₂/Si. The first silicon dioxidelayer is also referred to as the buried oxide or BOX. A second silicondioxide layer can be formed on the SOI wafer and polysilicon or siliconstrips can be formed on this structure to create the resonant cavity 106with the second silicon layer corresponding to the disk-shaped slab 205and the second silicon dioxide layer formed thereon corresponding to theinsulating transition layer. The thickness of this second silicondioxide transition layer can be controlled as needed. The polysilicon orsilicon strips can be patterned for example using photolithography andetching. Damascene processes are also envisioned as possible.

In the case where the substrate does not comprise silicon with a layerof silicon dioxide on the surface, a slab comprising crystal silicon canstill be fabricated. For example, crystalline silicon can be grown onsapphire. The sapphire may serve as the lower cladding for the slab.Silicon nitride formed for example on silicon can also be a cladding fora silicon slab. The formation of the transition layer and the strip onthe silicon slab can be performed in a manner as described above.

Other conventional processes for forming layers and patterning may alsobe used and are not limited to those specifically recited herein.Employing conventional processes well known in the art is advantageousbecause the performance of these processes is well established. SOI andCMOS fabrication processes, for example, are well developed and welltested, and are capable of reliably producing high quality products. Thehigh precision and small feature size possible with these processesshould theoretically apply to fabrication of strip-loaded waveguides asthe material systems are similar. Accordingly, extremely small sizedwaveguide structures and components should be realizable, therebyenabling a large number of such waveguides and other components to beintegrated on a single die. Although conventional processes can beemployed to form the waveguides described herein, and moreover, one ofthe distinct advantages is that conventional semiconductor fabricationprocesses can readily be used, the fabrication processes should not belimited to only those currently known in art. Other processes yet to bediscovered or developed are also considered as possibly being useful inthe formation of these structures.

One additional advantage of these designs is that in various embodimentselectronics, such as transistors, can be fabricated on the samesubstrate as the waveguide structures. Integration of waveguides andelectronics on the same substrate is particularly advantageous becausemany systems require the functionality offered by both electronic,optical, electro-optical, and optoelectronic components. For example,resonant cavities, modulators, switches, and other waveguide structures,can be optically connected together in a network of waveguides andelectrically connected to control and data processing circuitry all onthe same die. The integration of these different components on a singledie is particularly advantageous in achieving compact designs.

Another preferred embodiment for the resonant cavity 106 is shown inFIGS. 5 through 7. FIGS. 5–7 show the same views provided for theresonant cavity 106 illustrated in FIGS. 2–4, namely, cross-sectional,top, and perspective views. The cross-sectional view of FIG. 5 is acrossthe line 5—5 shown in FIG. 6.

The resonant cavity 106 illustrated in FIGS. 5–7 has a slab 304 like thedisk-shaped slab 204 in the resonant cavity shown in FIG. 2–4, however,this slab has a hole therein. Accordingly, the slab 304 is annular andmay be characterized as a channel-like waveguide instead of a slab-likewaveguide or as a hybrid of the two waveguide types. Nevertheless, thisportion 304 of the structure 106 will be referred herein as a slabregion with the understanding that it has a hole therein which may actto confine light within an annular region. The slab region 304 sits atopa cladding layer 302 formed on a substrate 300. The slab region 304preferably has a higher index of refraction than the cladding layer 302.In various preferred embodiments, the slab region 302 comprises singlecrystal silicon (e.g., active silicon) and the cladding layer 302comprises silicon dioxide (e.g., buried-oxide layer). Othersemiconductors may be used provided they are sufficiently transparent inthe wavelength range of interest.

As shown in FIG. 5, this embodiment further includes an insulating layer306 at least partially covering the slab 304. This insulating layer 306preferably has a refractive index lower than the refractive index of theslab 304, so as to serve as an upper cladding layer confining lightwithin the slab. The insulating layer 306 preferably comprises silicondioxide. As discussed above, the insulating layer 306 prevents unwantedflow of electrical isolated conducting pathway that form part of thestructure 106. Those skilled in the art will recognize that otherinsulating materials such as polymers may be used in forming theinsulating layer 306. This insulating layer 306 may comprise a pluralityof sub-layers.

This structure preferably further includes an annular strip 308comprising a relatively high refractive index material substantiallyaligned with the annular shaped slab 304. This material preferably has arelatively high refractive index in comparison to that of the insulatinglayer 306 covering the slab 304. The material comprising the annularstrip 308 is also preferably substantially transparent and non-absorbingto the wavelength light for which the resonant cavity 106 is designed.Preferably, the strip material is partially conductive and may comprisedoped semiconductor. In certain preferred embodiments, the annular strip308 comprises doped polysilicon, which has a refractive index comparableto that of single crystal silicon. Alternatively, the annular strip 308may comprise single crystal silicon. Those of skill in the art wouldrecognize that other materials may be used for the strip 308. Thematerials preferably have a substantially high refractive index incomparison with the insulating material covering the slab 304. Althoughthe strip 308 is shown as having an outer perimeter substantiallyaligned with that of the annular shaped slab 304, in other embodiments,the two need not be aligned. Furthermore, the slab 304 may extend wellbeyond the strip 308 especially in cases where the slab is not circularor annular but is a sheet or layer of material or unpatterned bulksubstantially wider than the total spatial extent of the annular strip.

The strip 308 is separated from the slab 304 by a transition layer 316of the insulating material to prevent the flow of current between thestrip 308 and the 304 to thereby facilitate carrier accumulation anddepletion. This insulating material preferably has a lower index ofrefraction than the slab 304 as well as the annular shaped strip 308.This transition layer 316 preferably has sufficiently thickness suchthat the carriers do not traverse this barrier either through defects(e.g., “pinhole” defects) or by tunneling. Conversely, the thickness ofthis dielectric layer 316 is preferably not so large as to require alarge voltage to be applied to the device to generate or deplete thedesired amount of carriers.

As shown in FIG. 5, the resonant cavity 106 further includes a first(strip) electrode 310 electrically coupled to the annular strip 308. Asshown in FIGS. 6 and 7, the strip electrode 310 includes a portion thatis annular in shape that is electrically connected to a voltage source320. This embodiment further includes a second (slab) electrode 312electrically coupled to the top surface of the slab 304. As discussedabove, the slab 304 has a hole therein and is likewise annular with aninner and an outer diameter and respective concentric boundaries definedby inner and outer edges. Similarly, the annular strip 308 has an innerand an outer diameters and respective concentric boundaries defined byinner and outer edges. Preferably, the inner diameter of the annularstrip 308 exceeds the inner diameter of the annular slab 304. The slab304 extends radially inward beyond the inner edge of the annular strip308 so as to expose an annular-shaped top surface of the slab 304 forconnection with the slab electrode 312. This inner portion of the slab304 preferably includes a doped region 314 in contact with theannular-shaped portion of the slab electrode 312 so as to create anohmic contact between the resonant cavity 304 and the slab electrode312.

As shown in FIG. 7, the slab electrode 312 is also electrically coupledto the voltage source 320, allowing for the application of a potentialdifference between the strip and slab electrodes 310, 312. This voltagesource may be AC or DC. Alternate sources of power for creating anelectric field between the strip 308 and the slab 304 are also possible.As shown in FIG. 5, the first and second electrodes 310, 312 areseparated by the insulating layer 306 to prevent unwanted electricalcontact between them. The insulating material 306 is not shown in FIGS.6 and 7 in order to more clearly illustrate of the interior features ofthe structure. The strip and slab electrodes 310, 312 preferablycomprise a metal, although one of skill in the art would recognize thatother materials may be used such as for example, polysilicon orsilicide.

As discussed above with respect to FIGS. 2–4, the structure shown inFIGS. 5–7 may be manufactured using conventional fabrication processesincluding but not limited to SOI and CMOS technology. Deposition andpatterning techniques may include for example, sputtering, chemicalvapor deposition, etching, and damascene processes, which are well knownin the art of semiconductor device fabrication as well as fabricationmethods yet to be developed.

In various other embodiments, the shape of the resonant cavity may beconfigured differently. For example, the annular slab 304 shown in FIGS.5–7 may be narrower such that for example the inner diameters (as wellas outer diameters) of the annular slab and annular strip 310 aresubstantially the same. In this exemplary case, the slab 304 does notextends radially beyond the edges of the annular strip 308 so as toexpose an annular-shaped top surface of the slab 304. Electricalconnection is made elsewhere to the slab 304. The narrower width of theslab 304 may act to confine the optical mode laterally. As discussedabove, this annular guiding structures 304 may provide lateralconfinement and for this reason is like a channel type waveguide incontrast with a slab waveguide, confining light both in vertical andhorizontal directions, even without the presence of the annular strip308. This configuration is referred herein as “ring-shaped.”

In certain embodiments, the annular strip 208, 308 and slab 204, 304 mayalso be shaped differently so as to provide a closed optical path otherthan circular or annular. Other geometries for guiding light are alsopossible. In addition, the resonant optical cavity path may not becompletely closed but may include interruptions, for example, wherelight can escape or be coupled into or out of the resonant cavity. Inother embodiments, the optical path may not be closed at all, and maymore closely resemble a Fabry-Perot resonant cavity with reflectivesurface on opposite ends, the light propagating back and forth ratherthan round and round a closed optical path. As discussed above, theresonant cavities can be formed using waveguides such as for example,strip loaded, channel, ridge or rib, and slab. These resonant cavity mayalso be formed from photonic crystal band gap waveguides or may compriseother types of guiding structures known in the art or yet to bedeveloped.

The electrode configuration may also be configured differently. Theoptical resonator shown in FIGS. 2–4 includes a first electrode 210 thatforms an uninterrupted continuous ring above the perimeter region ofdisk-shaped slab 204 and annular strip 208. FIG. 8 shows a top view ofanother embodiment for an optical resonator 106 in which the firstelectrode 410 only forms a partial ring, extending around less than theentire circumference of the resonant cavity 404. Metal is absorbing andmetallization and/or salicide in the proximity of the guiding region mayinduce attenuation of light therein. Accordingly, it is desirable toreduce the interaction between the metal electrodes and the opticalfield to avoid or reduce absorption of light by the metal. Theconfiguration of the strip electrode 410 in FIG. 8 advantageouslydecreases the amount of light absorbed by this electrode. In thestructure shown in FIG. 8, the resonant cavity 106 comprises a diskshaped slab 404 and an annular strip 408 similar to the slab 204 andstrip 208 depicted in FIGS. 2–4. The strip 408 forms a circular closedpath substantially following the perimeter region of the slab 404. Inthis case, the outer boundary of the slab 404, hidden in FIG. 8, issubstantially aligned with the outer boundary of the strip 408 althoughalignment is not necessary. For example, the slab 404 may extend beyondthe perimeter of the strip 408 especially in other embodiments where theslab comprises a sheet of material having large spatial extent incomparison with that of the strip 408. The first strip electrode 410 iselectrically coupled to the strip 408 and the second slab electrode 412is electrically coupled to the slab 404. Preferably, the strip electrode410 is spaced from the slab 404 by the strip 408 in a similar fashion asthe strip 408 shown in FIGS. 2–4. The slab electrode 412 is separatedfrom the strip 408 by insulating material, e.g., oxide. The insulatingmaterial separating the strip 408 from the slab electrode 412,especially at the location where the electrode passes over the strip, ispreferably sufficiently thick to reduce or avoid interaction between theslab electrode and the optical mode within the strip. Separation is alsodesirable to avoid shorting the slab electrode 410 on the conductingstrip 408.

In FIG. 8, the slab electrode 412 corresponds to the slab electrode 212,312 shown in FIGS. 2–4 and FIGS. 5–7, respectively, permitting theapplication of a controllable voltage between the first and secondelectrodes 410, 412. Various other features discussed above inconnection with the previous embodiments, such as the presence of thedoped region 214 in the slab 204, may be present in this structure fromof FIG. 8 as well.

Other embodiments of the resonant cavity may include two or moreelectrode segments positioned above the perimeter region of the cavity.For example, FIG. 9 shows a top view of an embodiment of an opticalresonator in which the first electrode 510 includes three spaced apartsections substantially extending along perimeter portions of theresonant cavity. In FIG. 9, the resonant cavity 106 comprises a diskshaped slab 504 and an annular strip 508 similar to the disk shaped slab204 and annular strip 208 of FIGS. 2–4. The strip 508 forms circularclosed path substantially following the perimeter region of the slab504. (In this case, the outer boundary of the slab 504, hidden in FIG.9, is substantially aligned with the outer boundary of the strip 508. )The first strip electrode 510 is electrically coupled to the strip 508and the second slab electrode 512 is electrically coupled to the slab504. Preferably, the strip electrode 510 is spaced from the slab 504 bythe strip 508 as are the corresponding strips 208, 308 shown in FIGS.2–4 and 5–7. The slab electrode 512 is separated from the strip 508 byinsulating material, e.g., oxide. The insulating material separating thestrip 508 from the slab electrode 512, particularly at the locationwhere the electrode passes over the strip, is preferably sufficientlythick to reduce or avoid interaction between the slab electrode and theoptical mode within the strip. Separation is also desirable to avoidshorting the slab electrode 410 on the conducting strip 508. The stripelectrode 510 includes connecting portions, omitted from FIG. 9 forclarity, that electrically connect the three illustrated portions. Theseconnecting portions are spaced above the portions of strip electrode 510shown in FIG. 9, and are separated from strip 508 by insulatingmaterial. The segmented strip electrode 510 permit electric fields to beapplied to designated regions of the annular strip 508. Absorptionresulting from interaction of a segmented metal/salicide strip electrode510 and the optical field within the strip 508 are also reduced bydecreasing the area of interaction between the strip electrode and thestrip. Segmented electrodes are also compatible with conventional CMOSfabrication processes and designs which employ a plurality of vias downto, e.g., salicide layers.

The strip electrode 510 and the slab electrode 512 shown in FIG. 9, areconnected to a supply (not shown) to permit the application of a voltagebetween the strip and the slab. Various other features discussed abovein connection with the previous embodiments, such as the presence of thedoped region 214 in the resonant slab 204, may also be present in thisstructure as well.

C. Operation of the Optical Resonant Cavity

The operation of the optical cavity 106 shown in FIGS. 2–4 will now bedescribed. Under certain conditions, when the resonant cavity 106 ofFIG. 2 is disposed sufficiently close to a waveguide propagating light,such as the first waveguide 100 of FIG. 1, light from the waveguide maycouple into the disk-shaped slab 204 of the resonant cavity. Because thedisk-shaped slab 204 has a substantially higher refractive index thanthe upper and lower cladding 206, 202 above and below, light canconfined therein. Lateral confinement within the slab 204 is provided bythe annular strip 208 which defines a substantially circular path aroundthe perimeter region within the slab. Light coupled into the resonantcavity 106 propagates around this closed optical path partially withinthe slab and partially within the strip. The optical mode will likely bedistributed within the strip, the region of the slab substantially belowthe strip and in proximity thereto, as well as within the transitionregion 216 therebetween.

Light traveling on a closed path within the resonant cavity 204 caninterfere constructively or destructively with itself depending upon thelength of the closed path, the wavelength of the light, and theeffective index of refraction along that path. More particularly, thecontrolling relationship is between the wavelength and the optical pathlength (OPD) of the optical path in the resonant cavity, i.e., productof the physical length of the path and the effective index of refractionalong that path. Light traveling on paths for which the total opticalpath length is an even number of half-wavelengths will experienceconstructive interference; light traveling on paths for which the totaloptical path length is an odd number of half-wavelengths will experiencedestructive interference. Because of this phenomenon, the resonantcavity 204 contains one or more standing waves at certain frequenciesassociated with different modes.

It is generally known that the m^(th) resonant frequency, v_(m), of ageneric resonant cavity is given by

$v_{m} = \frac{m\; c}{n_{eff}l}$where m is the mode number (an integer), c is the speed of light in avacuum, n_(eff) is the effective index of refraction of the mode in theresonator, and l is the path length of a full round trip inside thecavity. This equation applies to optical resonant cavities in general.

For a resonant cavity having a circular optical path, the circumferenceof the cavity determines the resonant wavelength. For the resonatordepicted in FIGS. 2–4, the optical power for the optical mode isconcentrated in a narrow band around the perimeter portion of the slab204 beneath the strip 208 and in proximity thereto. Significant opticalpower in this optical mode is also present within the annular strip 208,and within the portion of the insulating material 216 located betweenthe strip 208 and the slab 204.

Modulation of the resonant frequency of the optical resonant cavity 106may be achieved by changing the effective index of refraction of thematerial comprising the perimeter portion of the cavity 204. Changingthe index changes the effective optical path length, n_(eff)l, thuschanging the resonant frequency as dictated by the above equation.

The effective index of refraction of a mode is proportional to the realrefractive index, n_(o), such that:n_(eff)=n_(r)n_(o)where n_(r) depends upon the geometry of the waveguide. The change inthe resonant frequency, Δv, due to a change in the refractive index, Δn,is given by:

${\Delta\; v} = {{- \frac{v_{m}}{n_{0}}}\Delta\;{n.}}$This equation applies a broadly to optical resonant cavities in general.

As discussed above, the refractive index of a semiconductor, such assilicon, is dependent upon the existence of free carriers within thesemiconductor, such that increasing the number of free carriers in aregion generally lowers the refractive index of that region. Conversely,decreasing the number of free carriers in a region raises the refractiveindex of that region. Thus, by manipulating the number of free carriersin a region of a semiconductor like single crystal or polycrysallinesilicon, the refractive index of that region may be controlled. Changingthe refractive index changes the effective optical path length of thecavity, n_(eff)l, and, by extension, the resonant frequency, v_(m).Accordingly, the resonant cavity can be tuned.

The density of free carriers in a region of the disk-shaped slab 204beneath the strip 208, and within the strip as well, may be changed viathe field effect by applying a potential difference between the stripand slab electrodes 210, 212. As used herein the term “field effect”corresponds to the effect exhibited in field effect transistors (FETs).Application of an electric field to a semiconductor junction causes adepletion of carriers near the junction. With continued application ofthe field, inversion may result wherein opposite type carriers areattracted to the junction and the depletion region. In this manner, thefree carrier distribution in the semiconductor can be controlled andvaried by applying an electric field to the semiconductor. This junctionmay be formed between the semiconductor and an adjacent insulator acrosswhich the electric field is applied.

In this case, applying a voltage between the strip and slab electrodes210, 212 through the insulating transition region 216 creates anelectric field that may cause electrons to be depleted at the topsurface of the slab 204 beneath the annular strip 208, and moreparticularly, beneath the insulating transition layer 216. Thisdepletion of electrons occurs in the case where the semiconductor isdoped n-type and the a polarity is appropriate to force these electronsaway from the junction. Applying additional voltage between the 208strip and the slab electrodes 210, 212 may cause inversion wherein holesare attracted and accumulate at the portion of the slab 204 beneath thestrip and the transition layer 216. The existence of the insulatingtransition layer 216 prevents the holes from flowing into to the strip208.

The existence of the insulating transition layer 216 allows for themanipulation of the optical properties of the resonant cavity 204 usingthe field effect, a variation of which is utilized in field effecttransistors (FET) technology, such as metal-oxide on semiconductor fieldeffect transistors (MOSFET).

The field effect enables modulation and/or control of the free carriersand free carrier density underneath the strip 208, precisely where theoptical mode is confined, thereby providing strong interaction betweenthe carriers and the light. Increasing the magnitude of the appliedvoltage increases the depletion or accumulation of either electrons orholes, depending on the polarity and doping, and other conditions.Accordingly, the effective index of refraction can be changed. Thisability to variably control the refractive index permits tuning of theresonant frequency of the cavity.

Because metals strongly absorb light, it is advantageous to keep thestrip and slab electrodes 210, 212 at a distance from the optical pathof the resonant cavity formed by strip 208 and slab 204. The strip 208,comprising doped semiconductor, provides electrical connection whileseparating the metal electrode 210 from a substantial portion of theoptical energy in the mode. An electric field may therefore be appliedto the desired light path while minimizing or reducing light absorptioncaused by strip electrode 210. Other transparent conductors canalternatively be inserted between the strip electrode 210 and theperimeter portion of the slab 204. To further protect against lightabsorption by metal, portions or all of the strip and slab electrodes210, 212 may comprise conducting polysilicon, rather than metal.However, when adding high refractive material near the strip 208 andslab 204, however, care must be taken to ensure that the resonatorremains single-mode. Lower refractive index conducting material maytherefore be preferred.

As discussed above, the transparent strip 208 also serves to confine thelight to an optical path along the perimeter of the slab and thereforedefines the optical mode as is illustrated graphically in FIGS. 10A–10D.FIG. 10A portrays an perspective view of the disk-shaped slab 204 ofFIGS. 2–4. FIG. 10B shows a cross-sectional view of slab 204. The arrowwithin slab 204 indicates that light may propagate throughout theinterior of slab 204. This light corresponds to the optical powerassociated with a number of different modes. As discussed above, anevanescent field penetrates beyond the boundaries of the core region204. FIG. 10C shows slab 204 together with the annular strip 208. Thearrows in FIG. 10C illustrate the lateral spatial extent of the opticalmode supported within the slab 204 with the addition of the strip 208above the perimeter portions of the slab. As discussed above, light isconfined to the portions underneath the strip 208. Confining the lightto the perimeter region of the slab 204 may prevent multiple modes frompropagating within the slab. The dimensions of the strip 208 and slab204 as well as the respective indices of refraction and that of thesurrounding cladding determine what modes are supported. Preferably,these parameters are selected to support single mode propagation withinthe resonant cavity 106.

FIG. 10D depicts the insulating layer 206 between the annular-shapedstrip 208 and the disk shaped slab 204. As shown, the strip 208continues to provide confinement of the light in the periphery of thedisk shaped slab 204 despite the presence of the insulating layer 206,if this layer is sufficiently thin.

As discuss above, the slab electrode 212 may be electrically contactedwith the slab 204 via an ohmic contact between slab electrode and acentral doped region 214 of slab. The doped region 214 of disk shapedslab 204 provides two additional advantages. As illustrated in FIG. 10D,this doped region 214 is preferably substantially located at the centerof the circular shaped slab 204 and the dopant added to the doped region214 preferably strongly absorbs light. Because this absorption isprovided in the central portion of the resonant cavity 204, light fromthe perimeter portion of the slab 204 that propagates to the dopedcentral region 214 is preferentially absorbed. The dopant added to thedoped region 214 also preferably lowers the refractive index of thecentral region of the slab 204, thus enhancing confinement of lightwithin the periphery of the slab. As a result of these two effects, thedoped region 214, like the strip 208 discussed above, promotesconfinement of light to the perimeter portion of the resonant cavity204. These design features can be used to prevent the higher order modesfrom propagating within the slab such that substantially all the opticalpower can be concentrated into the one optical mode traveling around theperimeter of the resonant cavity 204.

The remainder of the slab 204 including regions beneath the strip 208may also be doped p or n type so that the semiconductor slab isconducting. The dopant is at higher concentration at the center of theslab 204 to quench modes in that region. Depending upon the doping andother geometrical considerations, a positive or negative voltage may beapplied between the strip and slab electrodes 210, 212 in order tomodulate the refractive index of the optical path in the resonantcavity.

FIG. 11 shows another preferred resonant cavity 106 that allows formodulation of the free carriers in a resonant cavity through the fieldeffect. This structure includes a substrate 600 analogous to thesubstrates 200, 300 of the earlier embodiments, on top of which is alower cladding layer 602 like the layers 202, 302 in the embodimentsdescribed with reference to FIGS. 2–4 and FIGS. 5–7. This resonantcavity further comprises a disk-shaped slab 604, preferably comprisingsingle crystal silicon. Although this slab 604 is disk shaped, as inFIGS. 2–4, it may have other shapes and may, for example, be annular, asin FIGS. 5–7. An annular strip 608 is disposed over the disk-shaped slab604 along a perimeter region of the slab. First and second (strip andslab) electrodes 610, 612, analogous to the electrodes 210, 212, 310,312 of the earlier embodiments, may be used to apply an electric fieldinto the perimeter region of the slab 604. (A doped region, omitted fromFIG. 11 for clarity, is preferably included to create an ohmic contactbetween the slab electrode 612 and the slab 604.) An insulating layer606 covers the slab 604. This resonant cavity 106 further includes afirst thin insulating transition layer 616 that separates the strip 604from the slab 604 in a manner analogous to the transition regions 216,316 discussed above.

The resonant cavity 106 shown in FIG. 11, however, includes a secondthin insulating layer 626 on the strip layer 608. A gate 618 is formedover the second thin insulating layer 626. This gate 618 may be annularin shape to match the annular strip 604 layer below. This gate layer618, however, preferably has a width smaller than that of the striplayer 604, that is, the difference between the outer and inner diameterof the annular strip is greater than the difference between the outerand inner diameters of the annular gate layer. The first (gate)electrode 610 is connected to the gate 618. The second (slab) electrode612 is connected to the slab 604. The strip 608 preferably comprised ofpolysilicon and may alternatively be comprised of crystalline silicon.Other materials that are preferably conductive and have a highrefractive index in comparison to the surrounding insulating layer 606may also be used. The gate 618 also may be comprised of polysilicon orsingle crystal silicon. Other preferably conductive materials may beemployed as well. Materials having a lower refractive index than that ofthe strip 608 will be less likely to alter the shape of the optical modein the strip and slab 604.

The strip 608 serves to confine light laterally within the slab 604 in aregion below the strip and in proximity thereto. Thus, optical power isdistributed in this region in the slab 604 as well as within the strip608 and the thin insulating region 616 therebetween as described above.This embodiment provides the advantage that the field effect created byapplying a voltage between the first and second (gate and slab)electrodes 610, 612 will may influence not only the free carrierdistribution in the slab 604, also the free carrier distribution in thestrip 608. In the “whispering gallery” mode, there is significantoptical power both in the perimeter region of the slab 604 and in theannular strip 608. This design allows for the variable control of therefractive index in both of these locations. The free carrier densitybeneath the first thin insulating transition layer 616 between the strip608 and the slab 608 can be controlled as described above. In addition,with the configuration shown in FIG. 11, the concentration of freecarriers beneath the second thin insulating transition layer 626 and inthe strip 608 can be selectively altered.

The electron concentration can be controlled in the strip 608substantially independently, by electroding the gate 618 and the strip618 instead of the gate and the slab 608. A stronger affect on theelectron density below the gate 618 may be achieved in this manner. Inthis specific configuration the voltage is across the gate 618 and thestrip 608 and not across the strip and the slab 604. Otherconfigurations can be employed to yield different results. For example,two voltage sources can be utilized to provide independent variablecontrol of the carriers within the slab 604 and those within the strip608. One voltage source can establish a field across the first thininsulating transition layer 616 and another supply can induce a fieldthrough the second thin insulating transition layer 626.

As shown in FIG. 11, the width of the gate layer 618 is preferablysmaller than the width of the strip 608. This reduced width is intendedto reduce perturbation of the shape of the resonator mode due to thegate 618, which may comprise silicon, and have a substantially similarrefractive index as the strip 608, which may also comprise silicon. Asdiscussed above, however, the strip 608 need not be aligned with theouter edge of the disk shaped slab 604 and the slab may extend wellbeyond the strip. Moreover, the slab 604 need not be disk-shaped an maycomprise a wide planar sheet or layer of semiconductor material. Inaddition, the gate 618 need not be aligned with the outer edge of theannular strip 608.

The resonant optical cavity 106 shown in FIGS. 5–7 operates similarly tothe optical cavity of FIGS. 2–4. Light traveling on a closed path withinthe annular resonant cavity 106 can interfere constructively ordestructively depending upon the relationship between the optical pathlength of the closed path and the wavelength of the light. Lighttraveling on paths for which the total optical path length is an evennumber of half-wavelengths will experience constructive interference;light traveling on paths for which the total path length is an oddnumber of half-wavelengths will experience destructive interference.Because of this phenomenon, the structure forms a resonant cavity 106that resonates at certain frequencies.

Variable control of the resonant frequency of the optical resonantcavity 106 may be achieved by changing the effective index of refractionof the material in the annular guiding region. The density of freecarriers in a region of the resonant cavity may be changed via the fieldeffect by applying a potential difference between the strip and slabelectrodes 310, 312. Applying the appropriate voltage between theelectrodes 310, 312 creates an electric field causing either electronsor holes to depleted or accumulated at the top surface of the slab 304beneath the annular strip 308. These electrons or holes cannot freelyflow between the strip 308 and the slab 304 because of the existence ofthe insulating layer 316 between slab 304 and the strip 308. In thismanner the resonant frequency of the cavity can be tuned.

As discussed above, the resonant cavity 106 preferably supports a singleoptical mode, such as the “whispering gallery” mode. This objective maybe accomplished by having the width of the guiding region sufficientlynarrow that only one optical mode is guided. In one preferred design,the outer edge of the slab 304 can be used to provide confinement. Inaddition, the slab 304, in this case annular shaped, may be sufficientlynarrow, i.e., the distance between the outer diameter and the innerdiameter is sufficiently small, to prevent other modes from existing.Strong confinement can also be a provided by the sufficiently narrowannular shaped strip 308 disposed above the slab 304.

As discussed above, to facilitate application of an electric fieldwithin the slab 304, the slab electrode 312 is electrically connected tothe surface of the slab. Preferably, an ohmic contact is formed byappropriately doping the contact region of the slab 304. This highconcentration of dopant which may, for example, be concentrated towardthe inner portion of the annular slab 304, may also assist in confiningthe optical mode to a localized region on the outer portion of the slab304. The dopant may reduce the refractive index in the highly dopedregion thereby enhancing confinement or may absorb optical energyoutside the guiding region.

The dimension of the slab 304 and strip 308 in large part along with thematerial the associated refractive index, define what modes aresupported by the waveguide structure. These dimensions depend paritallyon the wavelength of light for which the resonant cavity 106 is designedto operate. Various embodiments may be designed for light having awavelength between about 1.3 and 1.6 micrometers. However, thesestructures are not to be limited to any particular wavelength orwavelength range and may be designed for microwave, infrared, visible,and ultraviolet wavelengths.

The thickness of the insulating transition layer disposed between thestrip 208, 308, and the slab 204, 304 depends on the materials and onthe voltage to be applied to effectuate the desired index change. Thewaveguide structures may be appropriately configured to suit thespecific voltage range and index change.

As discussed above, these structures may be fabricated fromsemiconductor material such as single crystal silicon and polysilicon aswell as dielectrics such as silicion dioxide. Other materials may alsobe employed. Moreover, other semiconductor and dielectrics may also beemployed. In addition, various metals may be employed to form conductivepathways although non-metal conductors are also suitable and may bepreferred in certain circumstances.

In addition, although the optical path is toward the outermost regionsof the slab 204, the optical path need not be limited to this locationon the slab. In other embodiments, for example, the slab may be largerand may not even be circular. A closed optical path, circular ornon-circular, may be provided by, for example, strip loading or byridges or ribs positioned elsewhere than on the outermost edges of theslab. However, more compact designs might be those depicted in FIGS.2–7.

As discussed above, the waveguide structures are not limited to anyparticular type, such as a strip loaded waveguide having a relativelylow index transition layer. Rib or ridge, slab, channel, andconventional strip loaded waveguide designs may be employed. Forexample, tunable resonant cavity can be formed from a ridge waveguidestructure comprising semiconductor. A thin insulating layer can beformed over the ridge and metallization can be deposited on the thininsulator to form an electrode. The semiconductor can also be electrodedand a voltage applied between the preferably doped semiconductor ridgeand the metallization atop the thin insulating layer. The electric fieldthrough the thin insulating layer will induce the accumulation ordepletion of free carriers in the semiconductor ridge altering itsrefractive index. In this manner, the index of refraction of a ridgewaveguide can be manipulated.

Similar designs can be implemented for slab, channel, and stripwaveguides comprising semiconductor. Namely, a thin insulating layer canbe formed over these waveguides and metallization can be deposited onthe thin insulator to create an electrode. Applying a voltage to themetallization and preferably the doped semiconductor may cause electronsor holes to be depleted or accumulated in the semiconductor altering therefractive index therein.

In these designs, the metallization within close proximity to thesemiconductor waveguide may interact with the optical mode absorbingoptical energy and introducing attenuation. Crystal or polycrystallinesilicon can be substituted as an electrode material, however, the indexof this material may be sufficiently high and may perturb the opticalmode, depending on the particular design. The shape of electrode maytherefore be specifically shaped to yield the desired result.

Other configurations are considered possible and may be more suitablefor specific applications. For example, photonic bandgap crystalwaveguides may be used, however, the dependency of the index ofrefraction on carrier density may depend on a number of factors.Nevertheless, the usable waveguide structures are not to be limited tothose described herein and may include types yet to be discovered ordeveloped.

D. Operation of the Optical Switching Apparatus

The operation of an optical switching apparatus 104 incorporating theresonant optical cavity 106 of FIGS. 2–4 will now be described.Associated with the optical switching apparatus 104 is an opticalsource, preferably a laser. This light source is preferably a continuouswave (CW) source, although the operation of the switching apparatus 104is not so limited. The light output has a characteristic wavelength andoptical frequency determined by the optical source. The resonant opticalcavity 106 is designed to resonate at a frequency either at, or offsetfrom, the optical frequency of the light source.

The resonant cavity 106 may comprise resonators such as those describedor may comprise another type of resonant cavity. The followingdiscussion will assume for illustrative purposes that the resonantcavity 106 comprises the configuration of FIGS. 2–4. It should be isunderstood, however, that the discussion applies to other resonantcavities as well.

As described above, the resonant frequency of the optical resonantcavity is determined by the length of the optical path around thecircular guiding region of the slab 204 and the effective refractiveindex in this optical path. The dimensions and material of the resonantcavity 106 should be selected to create a resonant frequency close tothe optical frequency of the light source. Due to manufacturingtolerances, however, the resonant frequency of a particular resonantcavity is difficult to produce with sufficient precision. As such, aftermanufacturing, the resonant frequency of a particular optical resonantcavity may be adjusted, for example, through thermal tuning.

Thermal tuning refers to the manipulation of the resonant frequency ofthe cavity through control of the temperature of the cavity material.This tuning may be accomplished by thermally coupling a temperaturecontrol unit to the resonant cavity 106 that allows the temperature of aportion, or all, of the resonant cavity 106 to be adjusted. A Peltierheating/cooling system, for example, may be in thermal contact with theresonant cavity 106. Resistive or other heating or cooling mechanismsmay be employed as well to control the temperature of the waveguidestructure.

Raising the temperature of the resonant cavity 106 alters the resonantfrequency of the resonant cavity in two ways. First, thermal expansionof the disk-shaped slab 204 increases its diameter and the path lengtharound the perimeter. The resonant frequency of a particular mode of thecavity can thereby be decreased. Second, the increase in temperatureincreases the number of free carriers in the resonant cavity 106,decreasing the refractive index of the resonant cavity, and thusincreasing the resonant frequency. Because the latter effect is muchstronger than the former, increasing the temperature of the resonantcavity 106 increases the resonant frequency. Once an optical resonantcavity has been manufactured and tested, its temperature may be raised(via heating) or lowered (via cooling), as needed, to tune the resonantfrequency to the optimal frequency for a particular application.

Controlling the temperature of the resonant cavity can be employedinstead of or in addition to applying an electric field to alter thefree carrier density in waveguide structures and adjust or modulate theindex of refraction. Thermal tuning, however, may not be as fast astuning by using the field effect. In certain embodiments, thermal tuningwill be used to adjust the operating point of for the resonant frequencyof a resonant cavity and the field effect will be employed to rapidlymodulate the tuning.

In operation, an optical input from the optical source is propagateddown the first waveguide 100 shown in FIG. 1. Because the refractiveindex of the first waveguide 100 is much larger than the refractiveindex of the cladding region 108, the waveguide 100 propagates light ina guided fashion, as discussed previously.

When it is desired that the input signal remain in the first waveguide100 (i.e., to produce an output signal from the first waveguide 100 ),the optical resonant cavity 106 within the optical switching apparatus104 is set to a state where the resonant frequency is offset from theoptical frequency of the light source. Light of this optical frequencytraveling on a closed path within the annular resonant cavity 106interferes destructively therein. Accordingly, resonance is not achievedat this wavelength and light is not output from the resonant cavity 106,which as a result blocks coupling between the first and secondwaveguides 100, 102. In such a state, the light from the light sourcecontinues propagating down the first waveguide 100 without transferringany substantial amount of optical energy into the second waveguide 102.

Conversely, when it is desired that the input light switch to the secondwaveguide 102 (i.e., produce an output signal from the second waveguide102), the optical resonant cavity 106 within the optical switchingapparatus 104 is set to a state where the resonant frequencysubstantially matches the optical frequency of the light source. Asdiscussed above, this frequency shifting may be accomplishedelectronically by modifying the voltage between the first and secondelectrodes 210, 212, shown in FIGS. 2–4. The thermal state, i.e., thetemperature, of the resonant cavity 106 can also be changed so as toalter the free carrier concentration within the guiding region of theresonant.

The strength of the coupling between the first waveguide 100 and theoptical resonant cavity 106 will depend upon the spacing A between thewaveguide and the resonator as well as the dimensions and materials ofthe first waveguide 100 and the resonant cavity. Light from the lightsource traveling on the closed path within the annular resonant cavity106 interferes constructively therein. Accordingly, resonance isachieved. The cavity is filled with a high intensity electromagneticfield. Some of this electromagnetic energy is transferred from theresonant cavity 106 into the second waveguide 102 and is outputtherefrom. Accordingly, when the optical resonant cavity 106 is tuned tothe optical frequency of the light source, the light propagating in thefirst waveguide 100 can be strongly coupled or “dropped” into the secondwaveguide 102. The proportion of optical energy within the firstwaveguide 100 that is transferred to the second waveguide 102 depends ona number of factors such as the coupling efficiencies between the firstwaveguide and the resonant cavity 106, and between the resonant cavityand the second waveguide, as well as the absorption and scatteringlosses within the optical resonator.

Preferably, the relationship of the first waveguide 100 with respect tothat of the optical resonant cavity 106 is designed so that the all ofthe optical energy from the first waveguide 100 is transmitted into theresonant cavity 106 when on resonance. Under this condition, known as“critical coupling,” light coupled back from the resonator into thefirst waveguide 100 destructively interferes with the remaining lightpresent in the first waveguide 100. As such, no energy is output fromthe first waveguide 100. Instead, the optical power is filly transmittedinto the resonant cavity 106, where it is lost through two sources: 1)scattering/absorption in the resonant cavity 204; and 2) coupling oflight into the second waveguide 102.

The power coupled into the second waveguide 102 will necessarily belower than the power introduced in the first waveguide 100, due to thescattering/absorption within the resonant cavity. However, the magnitudeof the electromagnetic field strength in the second waveguide 102 (i.e.,the output signal) will be roughly proportional to the electromagneticfield strength in the first waveguide 100 (i.e., the input signal), withthe proportionality constant determined by the sizes of the losses inthe cavity.

There are different ways in which the system may be configured totransmit the optical power to either the first waveguide 100 or secondwaveguide 102. For example, the system may be designed so that applyingthe modulation voltage increases the frequency of the resonant cavity106 to the optical frequency from a lower starting frequency.Conversely, the system may be designed so that applying the modulationvoltage decreases the frequency of the resonant cavity 106 to theoptical frequency from a higher starting frequency. In anotherembodiment, the resonant frequency may match the optical frequency whenthere is no applied electric field in the cavity.

As discussed above, the introduction or depletion of free carriers in aregion has an effect on the absorption of light propagating through thatregion. As the free carrier density in guiding region of the resonantcavity 106 changes, the degree to which light is absorbed while passingthrough this region also changes. As such, when the carrier densityalong the optical path in the resonator is free of carriers, ordepleted, the absorption of light will be less than when free carriershave been accumulated. Conversely, when free carriers are injected alongthe optical path in the resonator, the absorption of light in theresonator is increased. This relationship holds true for most waveguidestructures. Photonic crystal band gap waveguides may vary differently.

As is well known, resonant systems may be characterized by adimensionless “quality factor” commonly referred as Q, where:

$Q = \frac{f_{0}}{\Delta\; f}$where f₀ is the resonant frequency of the resonator and Δf is thefull-width at half-maximum of the power spectrum of the resonatorsystem. The Q of a resonant cavity determines the field strength withinthe cavity. There is an inverse relationship between absorption in theresonant system and Q. As such, generally when the carrier density alongthe optical path in the resonator is free of carriers, or depleted, Q isincreased. On the other hand, when free carriers are injected along theoptical path in the resonator, Q is decreased.

The relationship between free carrier density and Q allows thecoefficient Q to be tuned simultaneously with the tuning of the resonantfrequency. FIG. 12 is a plot of power spectra for a resonator in twodifferent states. In the first state, the resonator is tuned to resonantfrequency, f₁ by accumulating carriers. This accumulation of carriersalso results in absorption and a lower quality factor, Q₁. In the secondstate, the resonator is tuned to a higher another resonant frequency,f₂, by depleting carriers. With less carriers and less absorption, thequality factor, Q₂ is lower. By selecting the size and composition ofthe resonant cavity, together with any thermal tuning, a particular Qvalue can be achieved at a desired frequency. For example, two resonantcavities of different dimensions can be designed to having identicalresonant frequencies and different Q values because one of them isthermally tuned to include more free carriers. This flexibility isadvantageous when the cavity is to be used as a filter where control ofQ is desirable. As discussed above, tuning can be alternatively achievedby applying an electric field to accumulate or deplete carriers as well.Thermal and electrical tuning can be utilized together as well.

When operated as an optical switch, it is advantageous that the densityof free carriers be reduced within the resonant cavity 106 when theoptical switch is coupling light from the first waveguide 100 to thesecond waveguide 102. With lower amounts of free carriers, losses due toabsorption can be reduced. Furthermore, it is advantageous that thedensity of free carriers be increased within the resonant cavity 106when the optical switch is not coupling light from the first waveguide100. In this latter case, it is desirable that the light continuepropagating along the first waveguide 100, with no power provided to thesecond waveguide 102, i.e., with reduced or negligible cross-talkbetween the two waveguides. By increasing absorption, losses in theresonant cavity 106 can be enhanced, and reflections from the resonantcavity back to the first waveguide 102 can be curtailed.

These conditions are accomplished by a configuration that decreases thenumber of carriers in the resonant cavity 106 when tuning the resonantfrequency of the cavity to match the optical frequency of the inputsignal. For example, the system may be thermally tuned to have aresonant frequency matching the optical frequency in the absence of anapplied voltage between the first and second electrodes 210, 212. Underthese conditions, light will be dropped down from the first waveguide100 to the second waveguide 102 through the resonant cavity 106 whenthere is no applied voltage from the voltage source 114. Applying avoltage then shifts the resonant frequency of the cavity away from theoptical frequency, removing the coupling between the first waveguide 100and the resonant cavity 106, thus switching the output signal to thefirst waveguide. Advantageously, carrier concentration and theconsequent absorption is less in the state where energy is dropped fromthe first waveguide to the second waveguide, than when the resonator isdetuned with the application of the voltage.

The relationship between the output signal on the first waveguide 100(“Output 1”) and on the second waveguide 102 (“Output 2”) 112 for such asystem is shown in FIG. 13. The modulation of the voltage appliedbetween first and second electrodes 110 is shown at the top. A voltageof zero (i.e., no applied field effect in the resonant cavity 204),results in a HIGH output in the second waveguide 102 and a LOW output inthe first waveguide 100. Conversely, when a modulation voltage isapplied (inducing the field effect in the resonant cavity 204), theoutput in the second waveguide 102 drops to LOW, and the output in thefirst waveguide 100 changes to HIGH.

Alternatively, the system may be designed to create a mismatch betweenthe resonant frequency of the cavity and the carrier frequency of theoptical source in the absence of an applied voltage. For such a system,application of the voltage shifts the cavity resonant frequency to matchthe carrier frequency, causing the output signal to drop to the secondwaveguide 102. For this system, the outputs on the two waveguides wouldbe reversed from what is portrayed in FIG. 13.

E. Modulating the Coupling Coefficient Between Optical Structures

In some applications, it is advantageous to modulate the couplingbetween two optical waveguide structures. The coupling may, for example,be controlled so as to preferentially allow, or preclude, propagation oflight from one optical waveguide structure to the other. If the opticalstructures are comprised of semiconductor, such as silicon, modulationof the free carriers in one or both of the semiconductor waveguides maybe used to manipulate the refractive index, thus altering theconfinement of optical modes therein and the coupling between thestructures.

FIG. 14A shows a pair of waveguides arranged to form a directionalcoupler. The waveguides include a core and cladding regions. The core issurrounded by the cladding material, not shown in FIG. 14, and as such,these waveguides may be considered channel waveguides although thewaveguides should not be limited to any particular type. The corepreferably has an index of refraction higher than that of the cladding.The core preferably comprises semiconductor such as for example singlecrystal silicon. The core may alternatively comprise polycrystalline aswell as other semiconductors which are preferably substantiallytransparent at the wavelength region of interest. In various embodimentsthese semiconductors are preferably doped. The surrounding claddingmaterial preferably comprises a dielectric such as silicon dioxide,although other relatively low refractive index insulators, such as air,may be used. The cladding in some cases may provide electricalinsulation between the two waveguide structures. FIG. 14B shows across-sectional view of a coupling region within of directional couplershown in FIG. 14A taken through the line 14B—14B.

Depending upon the relative refractive index of the core and cladding,the evanescent field associated with light propagating through thewaveguides will extend beyond the core different amounts. With lessconfinement, the evanescent field will continue farther outside thecore. Since the waveguides are within close proximity to each other, thespatial modes begin to overlap facilitating the transfer of opticalenergy therebetween. This effect is illustrated graphically in FIGS.15A–15C, which shows three plots of intensity versus distance along theline 15—15 shown in FIG. 14B. FIG. 15A shows the intensity for lightpropagating through each waveguide when the refractive index of the coreregion of each waveguide has a particular value, n₁. FIG. 15B shows theintensity when the refractive index the core region of each waveguide islowered, i.e., n₂<n₁. As shown in the figure, the lateral spatial extentof the evanescent field extends out further from the within thewaveguide, increasing the overlap between the two fields. FIG. 15C showsthe intensity when the refractive index of the core region eachwaveguide is raised such that n₃>n₁. In this case, the lateral extent ofthe evanescent fields shrinks, decreasing the overlap between the twofields.

The overlap between the two fields may be quantified by calculating an“overlap integral” that provides a measure of the strength of thecoupling between the two waveguides which depends on the shape of theoptical modes therein. The overlap integral may be used to determine a“coupling coefficient,” wherein the stronger the coupling between thestructures, the higher the associated coupling coefficient. Inspectionof FIGS. 15A–15C reveals that the waveguides associated with FIG. 15Bhave a larger coupling coefficient than the waveguides associated withFIG. 15C.

As discussed previously, the refractive index of a semiconductor, suchas silicon, may be varied by altering the distribution of free carriers.Likewise, the coupling coefficient associated with a pair of closelyspaced optical waveguide structures comprised of semiconductor may bemanipulated by altering the distribution of free carriers in one or bothof the structures.

FIG. 16 depicts a configuration for modulating the coupling coefficientassociated with the directional coupler of FIG. 14. As shown in FIG. 16,the first waveguide 700 and the second waveguide 702 may be electricallyconnected to a voltage source 708 via a first electrode 704 and secondelectrode 706, respectively.

Applying a voltage between the first and second electrodes 704, 706results in the capacitive storage of free carriers within the coreregion of each waveguide 700, 702. The capacitance between the twowaveguides permits storage of opposite charge on each waveguide, whichcan be employed to alter the refractive index of the waveguides. Theapplied voltage may for example induce free carriers of opposite sign ineach waveguide 700, 702. These carriers may be concentrated in the coreadjacent the cladding between the two waveguides 700, 702 but thischarge preferably extends through portions of the core where the opticalmode is distributed. The insulating cladding material prevents thecarriers from freely flowing between the two waveguides 700, 702.Application of the voltage creates an electric field, which may induceelectrons to accumulate in the first waveguide 700, and holes toaccumulate in the second waveguide 702, and vice versa, depending, forexample on the doping. Thus, the voltage may be employed to increase ordecrease the effective refractive indexes of either of the waveguides700, 702. The particular affect of the applied voltage depends on thedoping. For example, if the first and second waveguides 700, 702 are nand p doped, respectively, an applied voltage may be used to increasethe refractive index of both waveguides. Confinement is strengthened,shrinking the associated evanescent fields and thereby decreasing thecoupling coefficient. With a sufficiently large voltage, coupling withinthe waveguides can be reduced to a substantially zero. Changing the signof the applied voltage in this scenario decreases the refractive indexof both waveguides. The confinement is weakened, broadening the range ofthe associated evanescent fields and thereby increasing the couplingcoefficient such that the waveguides are more strongly coupled.

If the coupling regions of the waveguides 700, 702 are each n-type orp-type, applying a similar voltage to each will affect each waveguidedifferently: raising the refractive index in one waveguide whilelowering the refractive index in the other waveguide. With thiselectrical configuration, however, the refractive index in eachwaveguide is modulated substantially simultaneously.

FIG. 17 shows another configuration for modulating the couplingcoefficient associated with the directional coupler depicted in FIG. 14.In this design, the capacitance between the two waveguides and anotherclose-by structure permits storage of charge on the waveguides. Asshown, the first waveguide 720 and the second waveguide 722 areelectrically connected to separate voltage sources 727, 728 via a firstelectrode 724 and second electrode 726, respectively. Applying voltagesV₁ and V₂ between the waveguides 720, 722 and a nearby structure, suchas the substrate 729 of an SOI wafer, results in the capacitive storageof free carriers of the coupling region of each waveguide. Dependingupon the sign of the applied voltages V₁ and V₂ and the doping of thewaveguides (i.e., n-type or p-type), the refractive indexes in thecoupling regions of the waveguides 700, 702 may be independentlyincreased or decreased. As such, the confinement within the two guidesand the resulting coupling therebetween can be modulated as desired. Thecoupling coefficient will also vary accordingly.

FIG. 18 shows another configuration for modulating the couplingcoefficient associated with the directional coupler depicted in FIG. 14.As shown in FIG. 18, the first and second waveguides 730, 735 mayinclude both a p-type region 731, 736 and an n-type region 732, 737 soas to form p-n junctions. These waveguides are independentlyelectrically connected to voltage sources 734, 739.

The free carrier distribution associated with p-n junctions exposed toan applied voltage is very well known. In the absence of an appliedfield, some of the free electrons in the n-type region 732, 737 diffuseacross the junction and combine with holes in the p-type region 731,736. The region in which electrons and holes combine form a depletionregion 733, 738 lacking in free electrons and holes. The application ofa voltage across the p-n junction either expands or contracts the sizeof the depletion region 733, 738, depending upon the sign of the appliedvoltage. Forward biasing the p-n junction shrinks the depletion region733, 738 and, if the voltage exceeds about a specific threshold amount,e.g., 0.5 volts, depending on the design, a substantial electricalcurrent is created across the junction. Reverse biasing the p-n junctionexpands the depletion region 733, 738 and results in essentially noelectrical current unless the applied voltage exceeds a threshold“breakdown voltage” of the junction. When the breakdown voltage isexceeded, reverse bias creates a large electrical current across thejunction. Thus, for low power operation, this structure is preferablyoperated under reverse bias conditions below the breakdown voltage whenthe current through the junction is not as high as in the other modes ofoperation.

Applying a reverse bias to the p-n junction of one of the waveguides730, 735 depletes free carriers and increases the refractive index inthe waveguide thereby enhancing confinement and correspondinglydecreasing the evanescent field. With each p-n junction electricallyconnected to a different voltages, respective voltages can be appliedindependently to the first and second waveguides 730, 735. Thus, thecoupling coefficient can be modulated as desired. Various otherconfiguration are possible such as those discussed above, for example,wherein a single voltage source is electrically connected to bothwaveguides 730, 735 and/or electrical connections are made to a commonelectrical plane.

FIG. 19 illustrated how such coupling between two strip loaded waveguideportions having insulating transition layers can be controlled in adirectional coupler. The technique for modulating the couplingcoefficient between two waveguides portions is similar to that describedabove. As illustrated in FIG. 19, to form the directional coupler aplanar slab 744 is disposed on top of a cladding layer 742 formed on asubstrate 740. As shown, air may surround top portions of the slab 744creating a total internal reflection boundary at the slab/air interface.The slab 744 preferably comprises a material having a higher refractiveindex than the lower cladding layer 742 to, along with the slab/airinterface, provide the vertical confinement. Preferably, the slab 744comprises semiconductor which is doped and the cladding layer 742comprise a dielectric. In one preferred embodiment, the planar waveguidecomprises silicon, e.g., single crystal silicon, and the insulatinglayer comprises silicon dioxide, which forming the conventional SOIstructure discussed previously.

Light is confined horizontally within the planar waveguide 744 along twodistinct paths defined by first and second strips 748, 749, each extendlongitudinally to guide the light along curvilinear paths such as shownin FIG. 14. As discussed earlier, the high refractive index strips 748,749, have the effect of substantially confining the light to the regionsbeneath them. These elongated strips 748, 749 are preferably comprisedof polysilicon. Alternatively, they may comprise single crystal silicon.Other material may be used as well, as described above.

This structure further includes insulating transition layers 746, 747between the strips 748, 749 and the substantially planar slab 744 so asto allow for field effect manipulation of the free carriers in thewaveguides. A voltage can be applied through the first strip 748 via afirst strip electrode 750 electrically connected to a first variablevoltage source 760. A voltage can be applied through the second strip749 via a second strip electrode 752 electrically connected to a secondvariable voltage source 762. Each voltage source is preferablyelectrically connected to the surface of the substantially planar slab744 via leads 751, 753 that form ohmic contacts with a doped region(omitted from FIG. 19 for clarity) on the slab 744.

As discussed previously, the field effect may be used to control thedistribution of free carriers in a semiconductor, such as thesubstantially planar slab 744. Thus, by applying voltages V₁, V₂ acrossthe insulating layers 748, 747, it is possible to increase or decreasethe refractive index in portions of the slab 744 underneath the strips748, 749. Thus, the shape of the optical mode within the guiding regioncan be control, and the coupling coefficient can be modulated asdesired.

FIGS. 20A and 20B illustrated how the coupling coefficient between anelongated waveguide and a resonant cavity can be variably controlled.FIG. 20A is a top view of the elongated waveguide and resonant cavity.FIG. 20B is a cross-sectional view along the line 20B—20B in FIG. 20A.As shown, the waveguide and resonant cavity comprise elongated anddisk-shaped slabs 854, 804, respectively, disposed atop a lower claddinglayer 802 formed on a substrate 800. An insulating layer 806 is formedover the slabs 854, 804 to provide an upper cladding layer as well as toprovide electrical insulation for conductive pathways in the structure.The elongated and disk-shaped slabs 854, 804 preferably comprisematerial having an index of refraction higher than that of the upper andlower cladding layers 806, 802 to provide vertical confinement. Theslabs 854, 804 preferably comprise semiconductor, which may be doped,and lower cladding layer comprises dielectric material. Morespecifically, the slabs 854 and 804 preferably comprise single crystalsilicon and the lower cladding layer 802 preferably comprises silicondioxide, forming the SOI structure discussed previously.

An elongated strip 858 is formed over the waveguide slab 854 and anannular shaped strip 808 is disposed over the disk-shaped slab 804.Light is confined horizontally within the slabs 854, 804 by therespective strips 808, 804. The geometry of the structures are such thatlight will propagate longitudinally along the elongated slab 854 andaround a circular path around the periphery of the disk-shaped slab 804.The strips 808, 858 are preferably comprised of polysilicon or singlecrystal silicon. Alternatively, they may comprise other materials asdiscussed above.

Thin insulating transition layers 816, 856 separate the strips 858, 808from the slabs 854, 804 in order to allow for field effect manipulationof the free carriers in the elongated waveguide and the resonator. Avoltage can be provided through the strip 808 associated with theresonator via an electrode 810 electrically connected to a first voltagesource (not shown in FIG. 20). A voltage can be provided through thestrip 858 associated with the elongated waveguide via an electrode 860electrically connected to a second voltage source (also not shown). Thefirst voltage source also is preferably electrically connected to thetop surface of the disk-shaped slab 804 via an electrode 812 that formsan ohmic contact with a doped region 814 of the disk-shaped slab 804.The second voltage source is preferably electrically connected to thetop surface the elongated slab 854 via an electrode 852 that forms anohmic contact with a doped region of the waveguide 854.

By applying a voltages across the annular strip 808 and the disk-shapedslab 804, the refractive index underneath the annular strip may beincreased or decreased. Likewise, by applying a voltage across elongatedwaveguide 854 and the elongated strip 858, the refractive indexunderneath the elongated strip may be increased or decreased. Thus, thecoupling coefficient between the elongated waveguide and the resonantcavity 804 can be modulated as desired. Modulation of the couplingcoefficient can be implemented in addition to tuning the resonantfrequency of the resonator cavity.

In some applications, it is advantageous to maintain a constant couplingcoefficient (e.g., to maintain critical coupling) between a waveguideand a resonant cavity while manipulating the resonant frequency of theresonant cavity. The embodiment of FIGS. 20A and 20B allows for suchflexibility. In particular, as the refractive index in the resonantcavity is altered in order to shift the resonant frequency, an unwantedconsequence may be a shift in the coupling coefficient caused by theshrinking or expanding of the evanescent field in the resonant cavity.This undesirable consequence can be offset by manipulating therefractive index of the waveguide to maintain the same overlap integralbetween the fields in the waveguide and the resonant cavity. Thus, iftuning of the resonator shrinks the field in the resonator, thewaveguide can be simultaneously tuned to expand the field in thewaveguide so as to maintain a constant overlap integral. Conversely, iftuning of the resonator extends the field in the resonator, thewaveguide can be simultaneously tuned to shrink the field in thewaveguide so as to maintain a constant overlap integral. By tuning thewaveguide in step with tuning the resonant cavity, a constant couplingcoefficient may be maintained. As discussed above, this feature may beadvantageous in maintaining critical coupling which requires that thecoupling efficiency match the losses in the cavity. However, in otherapplications, the coupling coefficient may not need to be constant.

The structures and techniques for varying the effective index ofrefraction within a waveguide by altering the distribution of freecarriers therein can be applied to waveguide gratings 900 such as theone shown in FIG. 21. The waveguide grating 900 comprises a channelwaveguide 902 on top of a cladding layer 904 disposed on a substrate906. The channel waveguide 902 preferably comprises semiconductor, andmore preferably comprises doped semiconductor. Also, the channelwaveguide 902 preferably has higher index of refraction than thecladding layer 904. In one preferred embodiment, the channel waveguide902 comprises crystal silicon (e.g., active crystal silicon) and thecladding layer 904 comprises a silicon dioxide layer (e.g., aburied-oxide layer) on a silicon substrate 906.

An insulating layer (not shown) or a plurality of such layers may coverthe channel waveguide 902 on top as well as on one or more sides. Theinsulating layer preferably has a refractive index lower than therefractive index of the channel waveguide 902, so as to act as an uppercladding layer confining light within the channel waveguide. Theinsulating layer preferably comprises silicon dioxide, which has arefractive index substantially lower than the refractive index of singlecrystal silicon. The insulating material may comprise other materialssuch as for example silicon nitride and polymers, like polyimide. Theinsulating layer or layers also prevents unwanted flow of electricalcurrent between conducting elements of the device.

A plurality of strips or elongate members 908 are arranged over thechannel waveguide 902 to created a grating. In some preferredembodiments, the plurality of strips 908 comprise a material having anindex of refraction different from the insulating layer (not shown)formed on the channel waveguide 902 so as to perturb the effectiverefractive index of the channel at localized regions therein. Theplurality of strips 908 is disposed over but space apart from thechannel waveguide 902. Preferably, the strip material is substantiallyconductive and may comprise doped semiconductor. In certain preferredembodiments, the plurality of strips 908 comprise doped polysilicon orsingle crystal silicon, however, other different materials may be usedfor the strips 908.

The strip 908 is separated from the channel waveguide 902 by aninsulating layer 910 comprised of dielectric material to prevent theflow of current between the strip and the channel waveguide and tothereby facilitate carrier accumulation and depletion. This transitionlayer 910 preferably has sufficiently thickness such that the carriersdo not traverse this barrier either by tunneling or through defects suchas “pin hole” defects. Conversely, the thickness of this dielectriclayer 910 is preferably not so large as to require a large voltage to beapplied to the device to accumulate or deplete the desired amount ofcarriers. In one preferred embodiment, this transition layer 206comprises silicon dioxide.

As shown in FIG. 21, the waveguide grating 900 further includes stripelectrodes 912 electrically connected to the strips 908. Ohmic contactsand silicide may be used to create suitable electrical connectionsbetween the electrodes 912 and the strips, which preferably comprisedoped semiconductor. These strip electrodes 912 are electricallyconnected to a voltage source 914. This voltage source 914 may be an ACor DC voltage supply depending on the particular application. Some orall of the strip electrodes 912 may electrically connected together andto the voltage source 914. Alternatively, one or more voltage sourcescan be electrically connected to individual or groups of electrodes 912associated with different strips 908. The waveguide grating 900preferably includes one or more channel waveguide electrode 916electrically coupled to a surface of the channel waveguide 902. Onceagain, ohmic contacts and silicide may be employed to produce a lowresistance electrical connection between the channel electrode 916 andthe channel waveguide 902. The strip and channel waveguide electrodes912, 916 preferably are comprised of metal, although one of skill in theart would recognize that other materials, such as doped polysilicon, maybe used. Other configurations for establishing an electric field betweenthe strips 908 and the channel waveguide 902 are possible and should notbe limited to the electrical arrangement illustrated in FIG. 21.

As indicated by the arrow 920, optical power propagating longitudinallywithin the channel waveguide 902 is guided therein. As described above,the effective index of refraction within the channel waveguide 902 ishigher than the surrounding cladding regions, e.g., upper and lowercladdings 904, and is accordingly confined laterally therein. The strips902, however, may perturb the effective index of the channel waveguide902. The periodic perturbation of the effective index of refractioncreates a grating that scatters the light within the channel waveguide902. If the strips 908 are appropriately placed, light of the desiredwavelength will be coupled out of the channel waveguide 902 at aspecific angle, θ, as illustrated by arrow 922. This angle, θ, isdetermined in part by the effective index of refraction within thechannel waveguide 902 as well as by the spacing of the grating. Theserelationships are governed by well known principles of Bragg diffractionset forth in the following equation:

${\frac{2\pi}{\lambda}n_{eff}\sin\;\theta} = \frac{{\pm m}\;\pi}{\Lambda}$where Λ is the grating spacing frequency (i.e., the inverse of thegrating spacing), n_(eff) is the effective refractive index, m is thediffraction order, and λ is the wavelength of the light.

The electrodes on the strips 908 and the channel waveguide 902facilitate application of a potential difference between the strips andchannel waveguide. As discussed above, by applying an electric field tothe channel waveguide 902 through the insulating layer 910, the localdistribution of carriers below the individual strips 908 can be adjustedand controlled. For example, applying a positive voltage to a strip 908formed over a p-type semiconductor channel waveguide 902 will induce anelectric field that will cause depletion of majority carriersimmediately below the strip. Under certain conditions, inversion may beproduced wherein negatively charged carriers are attracted to thedepletion region. In either case, the free carrier distribution can becontrolled and varied. By altering the concentration of free carriers,the localized effective index beneath the individual strips 908 can beadjusted as desired. Accordingly, the effective index beneath one ormore strips 908 can be increased or decreased by application of theappropriate voltage to the selected strip. In this manner, the scattercross-section of the particular “ruling” or strip 908 of the grating canbe varied and controlled yielding either increased or decreasesscattering and resultant output coupling. In addition, by altering theeffective index of refraction, n_(eff), the angle of output, θ, or thewavelength at a particular angle, θ, can be altered and controlled as isset forth in the Bragg equation referenced-above. Switchable couplingand tunable filtering can be implemented in this manner. Otherapplications of controlling the carrier distribution within the gratingcoupler are also possible and are not limited to those discussed above.Increasing the electron density may also result in elevated levels ofabsorption, which may theoretically be desirable in certainapplications, and conversely, depleting free carriers may reduceadsorption.

Other waveguide grating configurations are possible as well. Thewaveguide grating 900 may, for example, be implemented using waveguidesother than channel waveguides. Ridge or rib waveguides, slab, and striploaded waveguides with or without low index transition layers are a fewexemplary candidates but the structures and designs should not belimited to these. In addition, the waveguides may have different shapesand may be integrated together with different structures. Differentmaterials and dimensions may be used. Finally, the grating design mightbe otherwise and may be altered depending on the application. Forexample the “rulings” or strips 908 can be shaped differently and mayhave other than rectangular or square cross sections. These grating mayfor instance be blazed. Still other variations in design are consideredpossible. In addition, in the case where the grating waveguide 900comprises a strip loaded waveguide having an insulating transitionregion as described above, it is possible to simultaneously change thedistribution of carriers beneath the strip (i.e., in the slab) andbeneath also the “rulings” or strips 908 of the grating.

The various structures discussed above, offer a wide range of advantagesand can be employed in a broad variety of applications. For example,tunable resonant cavities can be utilized for selectively filtering oneor more given optical frequencies. These resonant cavities may includean active material in guiding to provide gain and to thereby form alaser. Thermal or other types of drift in the output frequency of thelaser can be monitored and used as feedback to tune the resonant cavityin a fashion similar to that described above. These tunable resonatorscan be included together with a pair of waveguides to controllablycouple light from one waveguide to another. Switching can be performedin this manner. A light source can be modulated with such aconfiguration by directing light from the light source into one of thewaveguides. The output of either of the waveguides will correspond to amodulated optical signal depending on the modulation introduced by theresonant cavity. In this manner, information, voice or data, may beimparted on the optical signal. Switching can be implemented withdirectional coupler comprising two waveguides without any resonantfilter as described above. These filters, modulators, variable opticalcouplers, directional couplers and switches and various other devicesmay find use in optical communications and telecommunications but shouldnot limited to any particular application. Additionally, tunablewaveguide gratings can be created that allow the output angle,wavelength, and scattering strength, among other parameters, to bevaried and controlled.

As described above, silicon is substantially optically transmissive tocertain wavelengths of interest such as 1.55 microns. In addition,processes for fabricating silicon structures are well developed. Forthese reasons, waveguide structures comprising polysilicon and siliconare advantageous.

Although silicon is beneficial because it is substantially transparentat certain wavelengths, other materials and more particularly, othersemiconductors may be employed as well. Furthermore, the structuresdescribed herein are not to be limited to any particular wavelength orwavelength range and may be designed, for example, for microwave,infrared, visible, and ultraviolet wavelengths.

Those skilled in the art will appreciate that the methods and designsdescribed above have additional applications and that the relevantapplications are not limited to those specifically recited above. Also,the present invention may be embodied in other specific forms withoutdeparting from the essential characteristics as described herein. Theembodiments described above are to be considered in all respects asillustrative only and not restrictive in any manner.

1. An optical waveguide coupler having a tunable coupling coefficient,said coupler comprising: a first waveguide and a second waveguidecomprising first and second cores respectively, said first and secondcores separated by cladding in an evanescent coupling region, said firstand second waveguides supporting respective first and second spatialmodes, wherein said first and second waveguides are comprised ofsemiconductor having a distribution of free carriers, and said claddingcomprises a dielectric material that electrically isolates the first andsecond waveguides from each other, said first waveguide comprising anoptical path in a resonant cavity; a first electrode electricallyconnected to said first waveguide; a second electrode electricallyconnected to said second waveguide; and a variable voltage sourceconnected between first and second electrodes; wherein the distributionsof free carriers are responsive to application of said voltage such thatindex of refraction variations can be introduced in said first waveguideto increase or decrease confinement of said first spatial mode and alteroverlap of said first and second spatial modes in said cladding of saidcoupling region thereby changing the coupling coefficient between saidwaveguides from a first value to a second value.
 2. A method of tuningthe coupling coefficient of an optical waveguide coupler, comprising:providing a first waveguide comprising semiconductor containing adistribution of free carriers, said first waveguide comprising anoptical path in a resonant cavity; providing a second waveguideseparated from and electrically isolated from said first waveguide, saidsecond waveguide comprising semiconductor containing a distribution offree carriers, said first and second waveguides supporting first andsecond spatial modes respectively; and applying a voltage between saidfirst and second waveguides to alter the free carrier distribution insaid semiconductor so as to reduce confinement of said first spatialmode in said first waveguide and increase overlap between said first andsecond spatial modes, thereby changing the coupling coefficient of saidoptical waveguide coupler from a first value to a second value.
 3. Acapacitor comprising: a first capacitor electrode comprising a firstsemiconductor optical waveguide; a second capacitor electrode comprisinga second semiconductor optical waveguide, the second semiconductorwaveguide comprising a portion of a resonant cavity; a dielectriccomprising cladding disposed between said first and second opticalwaveguides, wherein the first and second semiconductor opticalwaveguides are sufficiently close to permit evanescent optical couplingtherebetween, said coupling changeable with application of a voltagebetween said first and second capacitor electrodes.
 4. The apparatus ofclaim 3, further comprising a variable voltage supply that iselectrically connected between the first and second semiconductoroptical waveguides.
 5. A method of optically switching, comprising:applying a voltage across first and second electrodes of a capacitorcomprising respective first and second waveguides separated from eachother by an inter-electrode dielectric wherein at least one of the firstand second waveguides comprises at least a portion of a resonant cavity;wherein application of said voltage induces storage of charge on saidfirst and second electrodes thereby altering coupling of optical modesin said first waveguide to said second waveguide.
 6. The method of claim5, wherein said voltage induces storage of opposite charge in the firstand second electrodes.
 7. The method of claim 5, further comprisingcausing index of refraction variations by inducing said storage ofcharge.
 8. The method of claim 7, further comprising modifying thepropagation constant of light in said first and second waveguides withsaid index of refraction variations.
 9. The method of claim 7, furthercomprising modifying optical confinement of said first and secondwaveguides with said index of refraction variation.
 10. The method ofclaim 5, further comprising modifying evanescent coupling between saidfirst and second waveguides by inducing said stored charge.
 11. Themethod of claim 5, further comprising transmitting an optical signalthrough said first waveguide and varying the fraction of light coupledfrom said first waveguide into said second waveguide by storage ofcharge in said capacitor.
 12. The method of claim 11, wherein saidvoltage applied between the first and second electrodes induces storageof opposite charge in the first and second waveguides.
 13. The method ofclaim 11, further comprising using said storage of charge to modify theeffective index of at least one of said first and second waveguides. 14.The method of claim 11, further comprising using said storage of chargeto modify the optical confinement of at least one of said first andsecond waveguides.
 15. The method of claim 11, further comprising usingsaid storage of charge to modify evanescent coupling between said firstand second waveguides.
 16. The method of claim 11, further comprisingusing said storage of charge to modify the propagation constant of lightin said first and second waveguides.
 17. The optical waveguide couplerof claim 1, wherein said first and second waveguides comprise elementsof a directional coupler.
 18. The optical waveguide coupler of claim 1,wherein said resonant cavity comprises a closed loop.
 19. The opticalwaveguide coupler of claim 1, wherein said resonant cavity is configuredto support a single optical mode.
 20. The optical waveguide coupler ofclaim 1, wherein said first and second waveguides are symmetrical abouta centerline disposed therebetween.
 21. The optical waveguide coupler ofclaim 1, wherein said first and second waveguides are monothicallyfabricated on a single substrate.
 22. The optical waveguide of claim 21,further comprising electronics integrated on the same substrate.
 23. Theoptical waveguide coupler of claim 22, further comprising dielectricmaterial over said first and second waveguides, said dielectric arrangedto provide electrical isolation for conductive pathways.
 24. The opticalwaveguide coupler of claim 1, wherein application of said voltageinduces free carriers of opposite sign to accumulate in the first andsecond waveguides.
 25. The optical waveguide coupler of claim 24,wherein electrons accumulate in the first waveguide and holes accumulatein the second waveguide.
 26. The optical waveguide coupler of claim 25,wherein the free carriers are concentrated in the first and second coresadjacent the cladding between the first and second waveguides.
 27. Theoptical waveguide coupler of claim 25, wherein the dielectric claddingmaterial prevents the carriers from freely flowing between the first andsecond waveguides.
 28. The method of claim 2, wherein applying saidvoltage induces free carriers of opposite sign to accumulate in firstand second cores in the first and second waveguides, respectively. 29.The method of claim 28, wherein electrons accumulate in the firstwaveguide and holes accumulate in the second waveguide.
 30. Thecapacitor of claim 3, wherein application of said voltage induces freecarriers of opposite sign to accumulate in the first and second opticalwaveguides.
 31. The capacitor of claim 30, wherein electrons accumulatein the optical waveguide and holes accumulate in the second opticalwaveguide.
 32. The capacitor of claim 30, wherein the free carriers areconcentrated in portions of the first and second waveguides adjacent thecladding between the first and second waveguides.
 33. The capacitor ofclaim 30, wherein the dielectric prevents the carriers from freelyflowing between the first and second waveguides.
 34. The method of claim6, wherein electrons accumulate in the first waveguide and holesaccumulate in the second waveguide.
 35. The optical waveguide of claim1, wherein at least one of the first and second waveguides comprises arib waveguide.
 36. The optical waveguide of claim 1, wherein at leastone of the first and second waveguides comprises a strip-loadedwaveguide.
 37. The method of claim 2, wherein at least one of the firstand second waveguides comprises a rib waveguide.
 38. The method of claim2, wherein at least one of the first and second waveguides comprises astrip-loaded waveguide.
 39. The capacitor of claim 3, wherein at leastone of the first and second semiconductor optical waveguides comprises arib waveguide.
 40. The capacitor of claim 3, wherein at least one of thefirst and second semiconductor optical waveguides comprises astrip-loaded waveguide.
 41. The method of claim 5, wherein at least oneof the first and second waveguides comprises a rib waveguide.
 42. Themethod of claim 5, wherein at least one of the first and secondwaveguides comprises a strip-loaded waveguide.
 43. An optical waveguidecoupler having a tunable coupling coefficient, said coupler comprising:a first waveguide and a second waveguide comprising first and secondcores respectively, said first and second cores separated by cladding inan evanescent coupling region, said first and second waveguidessupporting respective first and second spatial modes, wherein said firstwaveguide is comprised of semiconductor having a distribution of freecarriers, and said cladding comprises a dielectric material thatelectrically isolates the first and second waveguides from each other,said first waveguide comprising an optical path in a resonant cavity; afirst electrode electrically connected to the first waveguide; and avariable voltage source coupled to the first electrode so that a voltageis applied to the first waveguide via the first electrode; wherein thedistributions of free carriers are responsive to application of saidvoltage such that index of refraction variations can be introduced insaid first waveguide to increase or decrease confinement of said firstspatial mode and alter overlap of said first and second spatial modes insaid cladding of said coupling region thereby changing the couplingcoefficient between said waveguides from a first value to a secondvalue.
 44. The optical waveguide of claim 43, wherein at least one ofthe first and second waveguides comprises a rib waveguide.
 45. Theoptical waveguide of claim 43, wherein at least one of the first andsecond waveguides comprises a strip-loaded waveguide.
 46. The opticalwaveguide of claim 43, wherein at least one of the first and secondwaveguides comprises silicon.
 47. A method of tuning the couplingcoefficient of an optical waveguide coupler, comprising: providing afirst waveguide comprising semiconductor containing a distribution offree carriers, said first waveguide comprising an optical path in aresonant cavity; providing a second waveguide separated from andelectrically isolated from said first waveguide, said first and secondwaveguides supporting first and second spatial modes respectively; andapplying a voltage to said first waveguide to alter the free carrierdistribution in said semiconductor so as to reduce confinement of saidfirst spatial mode in said first waveguide and increase overlap betweensaid first and second spatial modes, thereby changing the couplingcoefficient of said optical waveguide coupler from a first value to asecond value.
 48. The method of claim 47, wherein at least one of thefirst and second waveguides comprises a rib waveguide.
 49. The method ofclaim 47, wherein at least one of the first and second waveguidescomprises a strip-loaded waveguide.
 50. The method of claim 47, whereinat least one of the first and second waveguides comprises silicon.